Paints and Coatings for External Timber Joinery - Session 2: Introduction to Paint Technology

Alison: Welcome, everybody, to this Technical Tuesday on paints and coatings for external joinery. So, I'm Alison Henry, and I'm Joint Head of Building Conservation and the Technical Conservation Team at Historic England. And I'd just like to introduce my co-presenters. So firstly, my job share partner, Domenico D'Alessandro.

Domenico: Thank you, Alison. Hello, everyone. My name is Domenico D'Alessandro. I'm Joint Head of Building Conservation with Alison, and I lead our Building Conservation Research Program.

Alison: Thanks, Domenico. And then, our guest speaker today is Peter Collins.

Peter: Hi, Alison. I'm all here and ready to go.

Alison: Okay. Brilliant. Thank you, Peter. So, before we make a proper start on the topic, I see there were a few people who are new to Technical Tuesday, so I just wanted to very quickly let you know a little bit more about the technical conservation team that we form part of. So we're a group of technical specialists, conservators, scientists, engineers and so on, and we lead technical research programs and develop guidance and advice on technical conservation issues. The first thing that Matt, I think, is going to paste into the chat there for you is about… It's a link to our main technical advice page on our website. And all our technical advice and guidance is accessible via that. Just to make it a little bit easier for you, we've collated all the published research reports and technical advice notes, and they're in that little brochure. And so, Matt's put a link to that too, and from that you can download all the PDFs and so on, and you can also sign up to our monthly newsletter if you missed the opportunity to do that at the very start of this webinar. And it really has only just once a month, and we don't inundate you with lots and lots of emails. It's literally just the technical conservation team updates, not wider HE news. So, moving on to today's topic, this is the first of- sorry, it's the second of two webinars on paints and coatings for external joinery. So last week we looked at repairing and preparing external timber joinery for repainting. So if you missed that, you'll be able to find that on the HE website, there’s a recording of that. And today we're going to focus on paint technology. It is very much focusing on the paint technology itself. I can already see the questions come in about removing bitumen from external oak. We're not going to cover paint removal because we are hoping to do another webinar in the autumn which will look at paint removal, and then reapplication of paint. So, we're going to keep it really focused on the technology of paints and paint systems. But anyway, before we do that, we wanted to tell you very briefly about a new major research program on paints and coatings that we've just embarked on. So, before Peter gets going, I'm going to hand over to Domenico, and he's just going to tell you a little bit about that research and how you can get involved. So, Domenico.

Domenico: Thank you so much, Alison. Just very briefly to introduce this research which we are carrying on at the moment. So, I’ll start with this painting which shows Allen’s Shop in St. Martin’s Lane in London. It’s by George Scharf, and it dates from 1829. And as you can see, this is showing us a house painter and his apprentice leaving the colour man with a fresh supply for the work, brushes, a jug of linseed oil, a keg of white lead, and a paint pot of ground paste freshly prepared by Mr. Allen. You can see him through the door. And then you can see him through the door, grinding the pigment. At the time of this painting, we are in the 1820s, there were a number of publications explaining how to use paint, which was being supplied in a form that is recognizable today, cheap oil paints for external purposes in a state ready to use. So, those years coincide with the rise of DIY, do it yourself. We all have a hundred brushes and paint in our lives, but needless to say, painting is a complex art. Tools, workmanship, substrate and paint technology, as we will see today, all play a part. Also, paint technology has gone a long way since then, and we often don't know exactly what is in the thing. For this reason, we have embarked on a research project, because there is limited independent guidance available on the topic of paints and coatings, and many meets have become current practice. Great attention is paid to the durability of the paint itself rather than the protection offered to the substrate. To address these concerns, we are undertaking, together with a range of partners and stakeholders, this research, which is titled, as you can see, understanding the properties and performance of historic paints and coatings, and identifying appropriate contemporary options for conservation. The meaning of the research is to provide evidence-based advice, as well as a robust methodology for selection and specification of paints and coatings.
At the same time, we would like to improve public perception of traditional external paints, increasing knowledge and developing capacity-building projects to address availability of suitable paint, coating materials, and skills.
Our areas of research broadly coincide with different types of substrates, external porous substrates and concrete, external timber, ferrous and non-ferrous metals, and then we have internal decoration.
We are posting relevant updates on our website in the conservation research section under materials and techniques. I think Matt has just posted the link in the chat. So, I'd just like to highlight that we have an active call for case studies for inclusion in our research pilot starting soon. We will be focusing on timber, external lime plaster and ferrous metal substrates, and you can take part by completing and submitting a short online form by the 26th of July. So, by the end of this week. Thank you.

Alison: Okay, brilliant. Thank you, Domenico. So, we're going to move on in a moment to the main topic of the day. Now, it's paint technology, and it's probably fairly logical then that it's actually going to be quite a technical webinar, quite a lot more technical than last week's webinar. And so, there may well be some terms that you've not come across before. Peter identified a very useful free online glossary produced by the International Standardization Organization. So, Matt has put a link to that in the chat. So it's a free online glossary. So if you make a note of any unfamiliar terms, you'll be able to use that glossary to check what they mean. So, I'm going to hand over to Peter now, who's going to present the bulk of the webinar. So, Peter is a paint technologist, and he's got over 45 years experience in research and development in the field of architectural coatings. And after a career in industry, he became the technical director for the Paint Research Association, who we've done research with, and he now runs an independent consultancy. So, I'm very pleased to welcome him properly and to ask him to present this webinar. So, Peter, over to you.

Peter: Thank you very much indeed, Alison. Really delighted to be here with you all today. There's so many people online. So, what are we going to try to do? Well, this webinar provides an introduction to paint technology for exterior joinery. So, let's… We've got this on bullet points. Never mind. So, we're going to try and look at why we use paints and the different types of paints that we find in the marketplace. As Alison said, to begin with, we've got an introduction to coatings technology, some of the definitions and glossary we use. We'll look at the paint raw materials and formulating relationships between them. We'll look at some of the numerous types of paint products in the marketplace and consider how the formulations vary and relate to each other. We'll look at the raw material components used in a paint formulation, focusing especially on the resin polymer binders. We'll look at standards for architectural paints, especially paints for exterior wood. I'll show you, as Alison has already alluded to, I'll show you a very useful paint glossary available free of charge before looking at the properties of paints and coatings. We'll look at the various standards that apply to all types of paints and coatings before concentrating in some detail on BN 927 series, which relates to coatings for exterior wood, which is of course the subject of this webinar. We will look at the various properties that are most frequently encountered on product technical data sheets before considering how to achieve excellent, long lasting finishes. And the issue of compatibility with previous coatings often gives rise to concerns. So, we'll look at this before moving on to consider, in some detail, what is probably the most important- definitely the most important aspect of achieving good durability, that is preparation and application.
And to wrap things up, I'll provide you with links to some useful reference sources before spending the remainder of our time answering any questions that you may have. They'll be compiled coming from the chat, submitted into the chat. And also, I think it's important to say that understanding the differences between period and modern joinery is crucial. Recognizing why our historic joinery has stood the test of time provides valuable insights and aids in making informed decisions about conservation and restoration efforts.
And this knowledge ensures that we respect and preserve the integrity of our heritage while applying appropriate, up-to-date techniques and materials. And so, as I say, some useful references. So, let's start by having a look at what we mean by paint. Put simply, paints are fully-pigmented opaque coatings, whereas alternatively we have transparent varnishes, clear coats, or semi-transparent stains, usually carried in organic solvent or perhaps water nowadays. And a stain is similar to a paint, but it contains less pigment, less resin and more solvent. Typically, it uses transparent pigments to enhance and bring out the natural grain of the wood.
So, next we look at why we use paint. Mostly to decorate and to protect structures. Decoration mainly relates to colour, but it also involves achieving an appropriate level of sheen or gloss, terms which are largely synonymous, with gloss being the wide ranging technical term, and sheen usually referring to lower glosses.
But also, there's the need for protection, and the needs for protection depending upon the nature of the substrate. Wood, masonry and metal have very different needs as building elements and building components, and as well as the decoration and protection coatings may also be applied to provide other functionality, such as indicating the contents of pipework in factories, fire protection in tumescent paints and varnishes, or more widely what are now often referred to as smart coatings. Things that respond to an external stimulus such as a temperature change in the environment, for example.
So, moving on, let's take a look now at what I might term wood's natural enemies. And the first of these really is to look at sunlight, because sunlight causes photodegradation, which in turn leads to the generation of free radicals. These are small chemical fragments that get broken off as a result of the photodegradation, and that in turn then leads to drying out of the substrate underneath, perhaps, if it's timber, which we're talking about, and shrinkage of that substrate. Rather like sunburn on our skin, the sun breaks down the cell structure of any exposed wood, producing free radicals that react chemically that cause further damage. And heat from the sun, as I've already alluded to, causes the wood to dry out, shrinking and cracking. On the other hand, moisture causes swelling of wood cells, again, chemically reacting with the wood as well, which enables those free radicals to move about causing yet more damage. It also leaches out products broken down by sunlight and water. And these, in turn, provide nutrition for microbiological organisms. Microbes. And that can be bacteria which are right on the wood. Some are already present in it, of course, and in the presence of water, they start to digest materials broken down from the wood or the paint, and they in turn provide nutrition for disfiguring black moulds and eventually wood rotting fungi take hold and also, of course, we are prone to algal growth on the surface, which commences in the presence of sunlight and moisture.
So we've got these things acting together, these factors that act together, defacement, movement and cracking, decay and rotting. And we put all those three together and we end up with a danger zone, as it were, with the combined activity of sunlight, microbes and moisture. So this then poses the question of what are we going to do to try to tackle that? What are going to be our principles of defence? And to defend against wood's natural enemies, a threefold strategy, I believe, is needed. So let's start looking at what that might be.
And we start off with timber selection, which is critical. So the species and grade of timber selected needs to be appropriate for the intended use. The timber itself must then be adequately seasoned to reduce its moisture content down from what can be as much as 200% or more to well below the fibre saturation point at about 30%, and further down below that to around about 10 to 15% moisture content.
Let me just check. I've got myself right on these clever drop downs I've done. No, no, not quite yet. So go back, that one. So let's go forward. Ideally, when we're producing timber, when we're cutting the timber, producing the lumber, the machining process would cut away most of the sapwood, leaving mainly heartwood, especially on exterior exposed surfaces.
However, that still leaves us with some difficulties and some problems to face, some challenges for which we need preservation. So all sapwoods and many heartwoods are perishable to varying degrees. In most timbers, the heartwood, that's the interior central core of the tree, is somewhat more durable than the sapwood, which is the outer surrounding cells around the heart, surrounding the heartwood. Those cells are actually there for the purpose of transporting moisture from the roots of the tree right up to the canopy, which is, of course, the sort of photochemical production unit of the tree, producing the cellulose in the materials and the good stuff that needs to get down to, to build and further strengthen that tree.
And the problem we face is that modern, often fast-grown plantation sapwood is often felled after only 40 to 50 years, and the sapwood is frequently used rather than just being discarded and therefore needs to be preservative treated, preferably by vacuum or pressure impregnation, in order to achieve reasonable durability.
So when redecorating, it's wise to apply a preservative prime. And we'll say a lot more about that in a moment. That contains a cocktail of biocides which effectively help to protect further against basidiomycete rots as well as actinomyces moulds. And these should be applied to the bare timber, especially into areas where rot has been cut out. And cutting out, I believe, is the best way to deal with rot. And also putting it into the glazing rebates, and I’ll say more about this in a moment, prior to painting with an undercoat. And then, finally, the last point on this dropdown is, a coating system should be applied to control the moisture content in the wood and to protect its surface. The coating system must be flexible enough to cope with any moisture movement coming from the wood, and it should contain biocides to kill off those surface moulds and algae that will alight on the surface of the coating. It needs to resist fading coming from that sunlight to maintain the aesthetic appearance of the building. And when all of these factors are taken properly into account, surface displacement is eliminated. Cracking and splitting is reduced, the tendency to anyway, and rot is prevented. And lo and behold, I’m pleased to say that leads us to achieving durability, good durability.
So now let's take a look at the sorts of categories of products that are on the marketplace that you'll find out there, that we see out there on sale. So, among these, we have solvent-borne products, and these range from things like oils through to varnishes, through to wood stains and paints. And so, architectural coatings, we can classify them, as I say, into solvent-borne or waterborne products, depending on whether an organic solvent such as white spirit or water was used to thin them and clean up the tools afterwards.
And included under this heading of solvent-borne products, we've got these oils. Linseed oil may not need sitting with any organic solvent as it is probably already pretty low in viscosity. Solvent-borne and waterborne coatings are further classified, as I say, into varnishes, stains, paints. Varnishes are clear coatings. Interior varnishes are often based on polyurethane resins, whereas the exterior varnishes often contain phenolic resins, which have some inherent UV screening attributes already inherent in them. However, it needs to be understood clearly by you that clear varnishes will always degrade more rapidly than semi-transparent wood stains, and even more so than fully opaque paints. This is because in order to be clear, they must transmit light through the coating. And this, in turn, permits the surface of the underlying wood cells to degrade, leading to adhesion failure, moisture ingress, and fungal attack. And I’ll explain more about the differences between varnishes, wood stains, be they semi-transparent or opaque, and paints in a moment.
Let’s put the waterborne slide up as well. So again, here we've got varnishes, we have wood stains. And we'll say more about the different types of wood stains, but they range through from sort of zero build preservatives through to low build stains, often used as base coats, and the medium build and semi-transparent stains, which, as I say, are either semi-transparent or opaque.
Within the category that we describe as paints, we have primers on the coats, top coats, paints for walls and ceilings, and then we've got various sheen levels. Mattes, eggshell, satin, silk, semi gloss. Those names are often used interchangeably, though typically one might argue that the semi gloss is the higher end of the gloss there, and then the gloss, and possibly we might even say high gloss finishes even beyond gloss finishes.
So let's take a look now at the paint components. So, turning now to the raw material components in the paint that most paint formulators use, we start off, not too surprisingly, perhaps, with pigments. And pigments are responsible for providing colour and opacity. Although some pigments may be quite transparent, as I've already alluded to, many pigments are based upon carbon chemistry, and these are what we term organic pigments. Nothing to do with them being organically grown. It's just simply, organic chemistry is what we call the chemistry relating to carbon chemistry. And these are typically used to achieve strong, bright colours. And they are distinguished from the inorganic pigments, such as iron oxides, which provide the earthy reds and yellow shades.
And as well as those prime pigments, the organic and inorganic, we also have a whole whole raft of so-called extenders included in the pigment family, mostly based upon natural minerals such as china clay, brown marble, chalk, talc, etc. They are used to reduce the gloss of paint formulations, and also to provide reinforcement and wear resistance, which of course is especially necessary for interior coatings. They also have the virtue that they're also usually much cheaper than the prime organic or inorganic pigments. So, the more of those that the formulator can put into the coating, the more likely the cheaper the formulation.
The polymers, these are also known as resin binders, and they're used, as the name implies, to bind the pigment together, giving the paint film cohesive strength, and adhesive performance as well. The adhesive performance needed to stick it onto the substrate. Several different types of resin binders are commonly used in architectural paints, but more about these on the next slide. But before we move on to the next slide, we also have to consider the thinners, water or organic solvents that are used.
And last of all, and not to be not to be forgotten, come the additives. There are many minor additives that may be included, such as wetting agents, thickness catalysts used to speed up the drying. And to put it this way, these are rather like the herbs and spices of the paint recipe. They’re usually added in small quantities, in small amounts, but they have large effects on the paint's properties.
So, let's turn now to look at the different types of resin binders that you're likely to find out there that are used typically. The main types of organic polymers used in resin binders and organic coatings are drying oils such as linseed oil, which, as you can see, or perhaps you may be able to see from the little diagram there, contains three long chain fatty acids. Unprocessed, raw linseed oil is quite thin, it's low in viscosity and usually requires no thinners to make it workable. However, because it contains relatively small molecules, around about 280 units in total, it takes time for these to knit together chemically to form a dry film. And the linseed oil, in order to speed that process up somewhat, may be processed by heat treatment, and then also blended with the raw, unprocessed linseed oil as in a stand oil to help speed up the drying process. Also, because of the large number of chemical double bonds present in the linseed oil, it's very prone to yellowing, and especially when used indoors, where it's shaded from the bleaching effect of the sunlight, lighting it down again.
Moving on now to the alkyd resins. These were discovered and developed in the 1920s to provide improved drying and hardness compared to linseed oil. They also contain fatty acids like those in the linseed oil. But more typically, the fatty acids would come from soybean oil. And so, they have quite a nice high bio sourced content. However they are synthesised as much larger molecules. The linseed oil, if you may recall, I said was about 280 units. These fellows are about 10,000 units, and consequently they become more viscous and require thinning, usually with white spirit. But then after the white spirit is gone, as the product dries, they harden much faster than linseed oil would, and yellow much less than linseed oil does.
And then, the last type of binder polymer that's there are the so-called acrylic resins, which are synthesised from petrochemical-based monomers, although nowadays some bio source acrylics are becoming available, but quite expensively so. These are typically extremely large molecules. Now we're talking several million units, and thus are not readily soluble in any solvents. So they are synthesised by a process known as emulsion polymerization, which forms particles. A polymer suspended in water, rather a bit like droplets in milk. And when the water evaporates, these particles coalesce together to form a hard, dry film. They fuse together. They merge together.
And I haven't really referred to them too much on the slide, but also, of course, polyurethane resins that we've spoken of earlier, used often for interior varnishes. These are modified alkyds, alkyds that are modified by [26:30 inaudible] technology, giving much harder films for use in interior varnishes.
So let's turn now to look at the relationships between the pigments, the polymer, and the solvents. The best way I know of explaining this is to use the so-called triangular diagram. And on this triangular diagram you'll see pigment down in the bottom left corner, polymer in the bottom right corner, and thinners there at the top. And actually what's happening here, if we did it numerically, which we won't do, you'd be going from 0% of pigment all the way along this thinners-polymer line. And you'd go up to 100% pigment down at the pigment apex. And in the same way you'd have 0% of thinners in the pigment-polymer line, and you go up to 100% of thinners up at the thinners apex. And then, in a similar fashion, on the thinners-pigment line, this axis here is no polymer present. And as we move across here, we increase in polymer content till we get to 100% polymer at this polymer apex.
So, at the top, we've got so-called low-solid systems, the solids being the pigment and the polymer content. As we come down from the top, down towards that bottom pigment polymer axis, we move to high solids. And that is, if you like, the drying process that's taking place as the solvent evaporates. And the solvent, as I say, can be organic solvents or can be water. But either way, as the thinners evaporate off, we increase in solids as we drop down to the bottom. And then, within this triangle, we can place all of the different sorts of products that are in the marketplace, paints are usually somewhere in the middle, and quite a quite a broad area of formulation space. Moving out towards the edge, we would have varnishes. And of course, clear varnishes are just on that polymer-thinners line itself. But oftentimes people will put a bit of colour into them and tint them. Hence, the reason I've drawn it is an ellipse moving inwards.
Moving on upwards, we would have so-called high-solids stains, and these are wood stains that have, relatively speaking, a lower content or thinners present in them. And then, moving across and inwards, and in fact putting more pigment into those high-solids stains, we get the so-called opaque stains. And we need the pigment there to make them opaque. And as we move onward and upward, we go firstly to low-solid stains where we've got, if you like, like the high-solids stains but thin down yet further with more thinners. And then, right at the top, we have so-called wood preservatives. And these are the sorts of things that really need to penetrate into the wood. So they need to be very, very thin, very low in viscosity to enable them to do that.
And then last, but by no means least, down here in the bottom left hand, left hand corner, we have so-called putties and mastics, the things that we use as glazing materials and so forth. Now, as we progress down that process going from low-solids to high-solids, the so-called drying axis, as the product dries, we move down along the bottom axis to the so-called dry film. And the dry films may be either matte, eggshell, semi-gloss, or gloss. And basically, as we're moving across there, as we move leftward, we're putting more pigment in, or that pigment may well be extender pigment, going into a matte paint or into an eggshell. And as we move across in the other direction, we're moving towards semi-gloss and gloss finishes.
And as already stated, the composition of the dry film is always located along the base of that triangle with the glossier, well bound formulations lying closer to the right-hand polymer side, and typically less than one quarter of the way from it. So quite close to the polymer side there. And formulations playing closer to the left hand side, conversely, the matt wall paints would typically be formulated at more than three quarters of the way away from the polymer vertex or one quarter away from the pigment vertex. And I think, Alison, you had a question you wanted to ask me.

Alison: Yes, also to just give you a little breather. One of the things that came up in our discussions earlier was about- you know, we often hear people say that traditional paints are inherently more permeable, than modern sort of alkyd resin paints. And that those modern paints can trap moisture in external joinery. But from what you're saying, if I understood correctly, it's not whether it is an old or a modern paint, it is the ratio of pigment to polymer that dictates that.

Peter: Yeah, yes, that's correct. The problem is that the lower the proportion of resin, then the less durable the paint will be as well. Because you need the polymer there to hold it all together and to hold it in place where you want it to be. So it's as simple as that. The other point really, I think there's already some questions that come up in the chat, but we'll cover them properly at the end. But you know, what's happening there is as we go from the left-hand side to the right-hand side, we're getting a decrease in what one might term permeance. And so the products are becoming less permeable because there's more polymer there that's binding everything together. And permeability is quite a complicated subject. And we'll cover a little bit more about that in a moment, when we're looking at paint properties as well. Shall we move on now? To consider the properties of paints both as liquid and as dry films. But I haven't forgotten about the issue of permeability and we'll come back to that. I’ll address that a bit in a moment.
So, let's start off by looking at the properties of liquid paints. Viscosity at the top there. That measures the consistency of the liquid paint. And we usually do that in units of poise or centre poise. A key thing to consider is that this can differ greatly when stirred in the can at low sheer compared to when it's spread by a brush at high sheer. A lot of paint products are sheer thinning. So the more you stir them, the harder you work them, the more that they tend to thin down. The density, that expresses the mass, the weight of a fixed volume of paint in grams per millilitre, or perhaps in kilograms per litre, depending on which units you choose to use.
Solids. This is the ratio of the dry film compared to the complete liquid paint expressed in either weight or volume percentage. VOC is the volatile organic content. It's the amount of solvent present in grams of solvent per litre of paint. The spreading rate states the area that you can expect to spread a volume of paint over in metres squared per litre. So, how many metres squared you’re going to cover with a litre of paint. And of course, you don't necessarily need to cover a whole metre square, but sometimes you might be covering a lot more than that.
Then of course, we have spray recommendations, which should include gun and pressure settings for different methods of spray. Cleanup. This provides us with some instructions on whether to use organic solvent or water to wash up our tools. And, you know, a big point that we're trying to really make in the paint industry at the moment is trying to encourage people to scrape the excess paint off of their paint brushes, off of their rollers. You can actually buy tools to scrape the paint off of the roller, and if you do that, you'll use much, much, much less water to clean your roller up, and have much, much less paint waste going down the drain when you wash up and clean up. The other top tip is to wrap your brushes in aluminium foil or cling film after you've used them, and then you can unwrap them the following day without needing to clean up in-between time. Especially, of course, if you're using the same colour, the same products. And that again, saves an awful lot of waste going down the environment and ending up eventually in our rivers and watercourses.
Drying time. That describes how long before a film becomes dry to touch, how long before it's unlikely to pick up dust or flies. Whereas the recoat expresses how long you should wait before applying another coat of paint. Gloss. This ranges from dead matte, to matte, to eggshell, to semi gloss, to gloss, and even to high gloss. Mattes are best measured at what we would describe as low angle. In other words, 85 degrees from the perpendicular. Almost looking along the wall is the best way to describe it, or along the window, whereas eggshells and semi gloss at about 60 degrees, so a bit of an angle away from. And the gloss at 20 degrees. So, almost perpendicular to the object you're looking at. And the reason for that is that at 20 degrees you'll be able to discern differences between products a lot more clearly and easily. It's easier to see differences in products that are high gloss at that very low angle, to the perpendicular, that is.
Colour. We usually measure that in terms of the so-called L*, A*, B* coordinates, where L* runs from darkness to lightness. This is the darkness-lightness of the paint product. The A* runs from greenness to redness, and the B* runs from blueness to yellowness. So, we end up with a three-dimensional set of coordinates that we can use to pinpoint any particular paint colour, and indeed to describe that paint colour. We talk in terms of the CIELAB coordinates of the paint product. We don't often find things such as adhesion and scratch resistance. Those are rarely quoted.
Now, permeability, as I've already alluded to, it’s a complicated subject. There are three terms to look out for, and you might want to go back over this after the talk as well in some more detail to take this on board fully. So water vapour transmission is measured in grams per metre squared over 24 hours. Typically, we measure that in a laboratory using a so-called paint cup, which is a little bit like a little bowler hat made out of metal onto which we can fix a painted circular disk with the paint on it, and we can put water into the little bowler hat and put it in a controlled environment, and then before we put anything in, we weigh the little bowler hat, as it were, and then we allow it to do its work over the next few hours, days, weeks, etc., and we wait at intervals and measure the weight loss from it. And that gives us the water vapour transmission in grams of water lost per metre squared over, say, a 24-hour period.
Now, permanence is the water vapour transmission divided by the vapour pressure at the temperature of measurement. And that's measured in grams per metre squared over 24 hours, but it's measured at 760 millimetres of mercury, which is normal atmospheric pressure. And the reason for that, of course, is that the vapour pressure, particularly the moisture vapour pressure in the atmosphere, will restrain, constrain the moisture coming out of that cup. And so, we need to know what the conditions are, both in terms of the temperature at which we're doing this measurement and the atmospheric pressure at which we're doing the measurement.
Then, moving on another step, permeability then is the permeance of the coating times the film thickness in grams per metre, again, measured over a 24-hour period at 760 millimetres of mercury. Now, why is this important? Well, if you think about it, the thicker the paint film, the lower will be the permeability. I mean, just in simple terms, if you have the same paint, and you put two coats of it, you're going to have something that's probably half as permeable as a single coat. And in turn, depending on the thickness of the film you put down in the first place, that will have a huge bearing on the permeability. So in simple terms, if the permeance is less than one when you measure it, then the coating is classed as impermeable. However, if the permeance is between one and ten, we would then classify it as being semi-permeable. And if the permeance is greater than ten, then we would classify it as being permeable.
Now this is the acid question, I guess. So most coatings will have permeances somewhere in the range of 2 to 5 grams per metre squared over 24 hours when measured at 760 millimetres of mercury. And that is actually quite suitable for moisture control in exterior joinery. So, in simple terms, most paints, the majority of paints, would be in the Goldilocks region, and will be just fine in terms of permeability. Domenico, did you want to ask me something about this?

Domenico: Very quickly, because I want to give you time to finish your slides and take questions. We hear a lot about micro porous paints. And so I like to ask you, are these really any good?

Peter: Well, I have to say, in my opinion, talk of microporosity is marketing spiel, and it really has very little bearing on exterior durability. It's worth remembering, of course, that if you're applying a new paint over existing coats of old paint, maybe multiple coats, the accumulated layers will already be pretty impermeable, because there's lots of it. So putting microporous paint on top really won't achieve any improvement on the situation for you.

Domenico: Thank you. Yes, very clear.

Peter: So let's move on to look at technical data sheets. Here are some points to consider when reading manufacturers technical data sheets: what type of product is it? What's the end use and appearance that's described? Does it claim to be exterior durable? EN927 is the thing to look for there, I think. Is it part of a coating system? And if so, where does it fit in? Primer, undercoat, topcoat? What's the colour range that's available? Is it in ready mixed or tinted paints? What's the stated gloss or sheen level? What type of thinner should be used with it? Is it water or solvent-thinned? And what is the solid content of the product, and what's the binder type? Oil, Alkyd, acrylic, etc.? What's the surface preparation and pretreatment that might be needed with the product? Should the product be diluted before use? And if so, to what extent? What's the recommended method of application? Brush, roller spray, etc.? What's the recommended spreading rate? How far out should one spread this stuff? What are the application conditions it can be applied under, in terms of moisture and minimum temperatures? What thickness of wet film, which will, depending on the solids, will lead to the dry film? And what are the drying times for the product, to get touch dry, hard dry, and recoat time?
There are a number of standards and specifications that I would encourage you all to look at, particularly BS 6150:2019, which I think all of you who are involved in specification should have a copy of at your desk or to hand available for you, and those various other EN927 series standards. I won't go into those at the moment because of time. You can see there are 15 parts in total. A lot of them. The ones to draw particular attention to is this natural weathering test. And also one, two and three, the classification and selection standards.
Classification. We do this by end use, appearance, and exposure conditions. We have various end use categories. Is the substrate a stable substrate, like a window frame? Is it a semi-stable substrate where some movement is permitted, tongue and groove cladding perhaps? Or is it a non-stable substrate like overlap, fencing, cladding, shed siding, and so forth? And that will have a large effect. The appearance categories. We have zero, low, medium and high build products. We have products that can be opaque, semi-transparent or transparent in nature. And then, the gloss that we've already referred to goes from matte to high gloss.
Then we need to look at the exposure conditions. What's the situation where this product's going to be used? Is the climate moderate, hard or extreme? Is the elevation itself sheltered, partly sheltered or non-sheltered? And that in turn leads to these various combinations in terms of the exposure conditions. And we can see here some of the impact of those different exposure conditions on products.
The performance specification here, EN 927-2 permits suppliers and end users to agree to a combination of tests, but they always should include these two mandatory tests. The natural weathering test, EN 927-3, and a moisture uptake test, EN 927-5. But there are also numerous other optional tests that can be achieved.
So, how are we going to achieve this excellent, long- lasting finish?It requires a combination of technology, care and attention and craftsmanship. It begins in the laboratory with the formulation of the product. Getting it right in the first place. Continues in the factory with the manufacture of the product. Requires knowledge on the part of you, the specifiers. It needs detailed inspection of the components that are going to be decorated, careful preparation of the substrate, skilful application, and only in appropriate conditions, and then timely inspection and maintenance in-between.
Generally speaking, compatibility is not a major problem. However, there are a few guidelines that you need to follow. Firstly, check the technical datasheet of the product you're going to be using to determine whether there are any specific issues that come with it. Try to ascertain the nature of the previous coating system, especially the topcoats, and be very vigilant for the presence of lead-based paints, especially on some of the older heritage properties that you're dealing with.
And for exterior woodwork, always use a product that's been designed for exterior wood. Never use masonry paints on woodwork unless, and I'm rather sceptical about this, unless they're claimed to be so-called multi-surface paints.
So, preparation and application. To begin with, always ensure that the moisture content of the wood is below 18% before commencing work, and you can measure that with a moisture metre. Avoid stripping back coatings to bare wood. It's rarely necessary. Remove any flaking or poorly adhering paint by scraping. Examine carefully for the presence of wood rot, cutting out any soft timber and replacing it. And before you replace it, treat any of those areas of rot after removal with a preservative primer. Replace the wood or fill any cavities with an exterior durable flexible filler. Because the wood's going to move whether you like it or not. Wet flat the surrounding paint work to feather in the edges, wash down thoroughly, and allow it to dry, and consider perhaps using a fungicide or algicide wash for this task. Always follow the manufacturer's instructions for the system generally, but apply the exterior primer undercoat to areas of bare timber. Apply exterior undercoat of the entire substrate, and apply an exterior topcoat over the entire substrate. And only apply paint in dry weather and if temperature will remain above that specified by the manufacturer during application and drying.
Here's an example of a window frame. And first, as I say, inspect, checking the weather conditions and the moisture content of the joinery, rake out the joints, defective glazing material, cut out the rotten wood, replace the rotten wood with new wood or using a flexible filler, scraping away any flaking or poorly adhering paint, wet flatting the surrounding areas and feathering in the edges, washing down a line to dry. Consider using a biocidal wash, and unless strictly necessary, avoid stripping back the entire window. This preservative primer, especially into the joints, contains a bar side and a polymer to seal the end-grain. Apply to all the bare wood, especially to those joint areas and glazing rebates. Put a flexible high build exterior undercoat over the top of that that's also got algal and fungal resistance built into it. And finally, a top coat that's colour fade resistant, and is fungal and algae-resistant.
Here are some useful resources. I'm sorry, I've run out of time. I've gone way over time. But some useful resources that you can have a look at afterwards with links to them in the presentation. So, over to you now for questions. If we have any time left. Alison, I hope.

Alison: I think we've got a few minutes. You suddenly speeded up at the end?

Peter: I needed to.

Alison: It's really fascinating. So, lots of questions coming in. I've sort of started trying to group some of those into different topics. So, we'll go through some of those now. We can start with one from… So, there was actually one from last week and we said we would hold that over and ask it this week, because it was a question about paint technology rather than one for Vincent last week. And it was about the difference between zinc oxide and titanium dioxide compared to lead in linseed oil paint for combating mould growth. So, we know that the lead is really good in the oil paints. And so the question was, well, how do you find zinc oxide and titanium dioxide for combating mould growth?

Peter: Okay, that's fairly straightforward. Titanium dioxide really has very little influence on mould growth whatsoever. It's a phenomenally good white pigment. It gives us really strong opacity for a relatively modest amount added into the product. But it's not bioactive in any way, really. It's photo active, and so it needs to be properly encapsulated and so forth. Zinc oxide, I would tend to describe that more as being a biostat, fungistat rather than a fungicide. It slows down and inhibits mould growth, but it doesn't tend to kill it off. Good old lead is of course very toxic. And it's not only toxic to human beings, but it's also toxic to microorganisms. So, it kills everything off. It's as simple as that. So, in terms of microbiological activity, lead is excellent, but it comes with some drawbacks, shall we say, in terms of…

Alison: And it’s hardly available now to people. We've got a few questions around the issue of breathability. So, it was a question about determining the breathability. Many products claim to be breathable, but is guidance on the minimum vapour permeability that a paint should provide? Or is it that red herring…

Peter: Both. I mean, it's been hyped up. That's the point. You know, marketers are looking for something really snazzy to say about their products. They talk about micro porosity. There's a little bit, in the sense that… Shall we say, liquid water and water vapour or rather different in their physical nature and behaviour. However, for the most part, what's coming out of windows and joinery is the moisture vapour coming out, and what we're trying to stop getting in is the liquid water, which, you know, runs off and sheds off for the most part, other than in the cracks and in the corner joints where it creeps in and down the glass, as it were, behind the coating. But it comes back to this business of permeance. You know, if the permanence is less than one, then we've got something that's extremely permeable, and is liable to let water in as much as it stops water from going out. And on the other hand, when you get above ten in terms of permeance, you've then got something that's extremely impermeable. And yes, you know, if you're up in that range, then you've got a pretty sealed system. And in a way that might not be too bad, except for the fact that cracks will occur, and when those cracks occur, then of course, the water finds a way in. And when it finds a way in, then the next thing we get is expansion. We then get stress on the coatings, and furthermore, with the water coming in, we'll get biological organisms with it too. And the next thing we know, we've got rotten windows on our hands. So, it is important to be somewhere in what I call the Goldilocks zone, in the middle there.

Alison: Someone so she asked the question about how trapped moisture can evaporate through paints that were in that Goldilocks zone.

Peter: How it can evaporate through? Well, I mean, basically, you've got a transportation mechanism on there. Materials always will tend to move from areas of high concentration to areas of lower concentration. And, you know, it's just the nature of things that things migrate. So, you know, be it through osmosis, on the one hand, or just simply through evaporation. The moisture will migrate through the coating, working its way through channels in the coating. And that's why I said about the pigment to polymer ratio, because if you've got a lot of pigment there, then you have channels between that pigment that allows the moisture to transit through and out. And of course when it then gets onto the top surface, it evaporates. And so that makes room for more moisture to come through. So you've got this sort of evaporative process driving and taking the moisture away, and heat as well, driving that process too, in terms of the energy.

Alison: So this ties in with what you'd said about Someone had asked a question about the differences in protective qualities of paints with different finishes. I think you've already covered this because you talked about eggshell, saying that the glossier paints are less permeable, but the eggshells might be more permeable, but possibly less durable.

Peter: And also, you know, the binder type can have an impact too. So, you know, again, without going into great detail, but polymer binders that have started off in life as dispersions and then fused together, you often get little pathways that moisture can migrate through. So typically, typically acrylic, acrylic latex paints would tend to be on the more permeable side of things as compared to, say, an alkyd paint. The linseed oil paints, there's a lot of binder there. It's cross-linked together. There's not really going… Depending on the amount of pigment you put into it, there's probably not going to be a lot of difference between a linseed oil based paint that's fully dried and cured, if it's got the same level of pigment in it, as an alkyd pipe fully dried and cured. They'll be fairly similar to each other at the end of the day.

Alison: Yeah, that's interesting. Thank you. Then we've got a few questions relating to durability. And one was about- you mentioned that the linseed oil yellows, and somebody was asking if they had heard that right. Well, I think they have, haven't they? But then that led on to some questions about whether artificial drying with UV lights avoided that. But I assume it's the long-term exposure that is going to cause that yellowing.

Peter: That's a slightly two-edged answer, in the sense that- what’s the right way of putting it? Yellowing occurs more in the dark than it does in the light, because sunlight tends to bleach the chemical compounds that are making it go yellow in the first place. So, in a sense, one could imagine that using UV light might well bleach some of that yellowness out. These are so-called shift spaces that are produced, come out as breakdown products from the cross-linking process of the linseed oil. And, you know, all fatty acids do it to some extent, but just some fatty acids do it more than others. And linseed oil is one of the worst. It's as simple as that, really, in terms of doing that yellowing.

Alison: So there's a few questions related to materials and products. And I just wondered, have you come across zero tension paints, and could you say something about those, why we might use those?

Peter: No, this has to do with surface energy, surface tension of the product. And this relates as well, I suppose, in a sense to water shedding properties. Products that have got very, very low surface tension will tend to cause water droplets raining on it to bead up and form droplets, which get heavy and then run off and drain off, whereas if you've got something that's quite high in surface energy, then the water then spreads on it. And again, it's a complicated, complicated subject, because there are those in this world who would argue that high surface energy is a good thing, and those that would argue that low surface energy is a good thing. And it really does depend on the situation and circumstances. The trouble is, if you have droplets beading up, they then evaporate off, and you often get circles left on your paint where they've been. And if you have low surface energy and the water runs off and sheds very easily- or high surface energy, sorry. High surface energy, low surface tension. The other way around. Under those circumstances, what you can have is that the dirt and stuff that's on there gets washed off. It comes away quite nice and easily. So there are people who would say, well, you know, having a paint product that's got a very low surface energy is a good thing. It stops the dirt build up and so forth through one mechanism, but then it causes problems through another mechanism. So I don't think there's a cut and dry answer to that.

Alison: Okay. I think it probably depends on the particular applications as well.

Peter: And things like the slope and runoff that you've got, you know? Can it shed the water easily?

Alison: Well, we are getting a bit short of time, probably time for a couple more. I won't ask questions that relate to specification because I think actually that's not the role of a paint technologist. So, we focus on the things that relate more to paint technology. So, there's a question about pigments, and are some colours more likely to fade in sunlight than other colours?

Peter: Yes, yes, yes, yes. The ones are renowned for being not good in terms of fading are bright yellows. And the way around that is to use very expensive yellow pigments, the sorts of things that are used on cars, because otherwise they fade. The general-purpose architectural grades of pigments that are affordable for use in architectural coatings do tend to be on the fading side. Going back to the past, lead chromates were great for that because they didn't fade at all. But again, lead. So, we can't use those.

Alison: And another question is about varnishes. This is a good question. Do you treat against fungi, the fungicidal treatment, before applying varnish as well as paint?

Peter: Definitely, definitely. Because it’ll run rampant underneath the varnish. And it often leads to this flaking and peeling, and black discoloration that we see on the varnish typically very often.

Alison: And do you have any views about which are more durable, whether water-based or oil-based varnish.

Peter: Varnish? Well, I'd say neither of them are very durable, to start off. Going back to the sunlight issue, and sunburn. Neither of them are gonna be very durable. The best bet is to put a yellow transparent oxide preservative on first as a base coat, if you like. And hopefully, if your wood is the right sort of colour, you can get away with putting a yellow base coat on before you put your clear varnish on top, and it doesn't change the colour too much. That will help, actually, in terms of- it’s got preservative in it, so it’s gonna help with the fungi, but it’ll also help with UV protection, photodegradation.

Alison: The final question… We didn’t cover anything about mineral paints and lime wash, I’ve just seen a few questions coming in, because that relates to a different substrate. So, we’re focusing just on timber, sorry about that. But just about faster-drying oil-based paints compared to standard slower-drying oil-based paints, are they more durable? I mean, in my experience, I must say, the oil-based paints all seem to dry quite slowly, and it’s the acrylic paints that are marketed as drying quickly. But perhaps there are some oil-based ones that dry faster.

Peter: If they use stand oil or linseed oil in them, then they will dry a bit faster, because you've increased the size of the molecules. You've joined the molecules, you've knitted some of the molecules together before you start applying. So, they will dry a little bit faster for you.

Alison: Does that affect durability, I suppose, is the crux of your speed?

Peter: Again… If what you're looking to do is to apply a system and leave it, with washing and some maintenance, but with minimal maintenance over a long period of time, then I wouldn't be using oil-based paint, if that makes sense. I'd be using an alkyd paint through or an acrylic paint, because both of those will stay in place for a lot longer, I believe, than an oil-based one. Now, the oil-based products do come into their own if you are prepared to go back at regular intervals and reapply and maintain them. And it's very simple. If you do that, then they work fine. But if you're not going to do that, if you expect to put it on and walk away, and come back seven years later, well, you're going to come back to quite a mess, I’m afraid.

Alison: Yeah. I'm really glad to hear you say that, because I think that was one of the key takeaways from last week's webinar. You know, that maintenance is the cause of so much deterioration. And then people always blame the product. They're looking for products that will do away with poor maintenance or lack of maintenance.

Peter: Understandable. You know, it costs money to maintain.

Alison: So I think we'd better wrap it up. There were just a few things I wanted to add. So actually, Matt, could you post the links from the last slide? I'm not sure whether- I've not been watching the- I've been watching the questions, not the main chat. So I don't know if there's a chance, while we're just wrapping up, all those links, that Peter put…

Matt: I can do that for you, yes.

Alison: Thank you. Yeah. And then, one other thing, just to let you know that we are working on, but it is going to be quite some time. I said we weren't going to answer questions about paint specification, but Domenico and our colleagues in the paints and coatings research, we are developing a sort of decision making tool to help with specification. Because it isn't just about the paint, you know, there's so many more factors that need to be taken into account, and it is quite complicated, and it's something we get a lot of inquiries about. You can tell from the questions that we've been seeing in the chat. So, just to let you know, we don't have a timescale for that, but it is something that we're working on, and we're very, very aware that that will be, I think, a really useful piece of work in due course. So, do stick with us, and, you know, eventually we will get a lot more guidance on paints and coatings on our website, using the information from Peter and from Vincent’s webinar last week as well.

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