The Best Paints for Lime Plaster
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Last August we began our journey with Lime Plastering, and over the past 11 months have been gradually working our way through plastering the entirety of the Cottage. Last week marked another major milestone in the renovation, as the barn window in the Foyer of the Cottage was replaced with a beautiful new frame, handmade by James Wilding and his team at James Wilding Joinery. Not only did they do a fantastic job, and I’m sure you’ll agree that the window looks gorgeous, but with the new frame in situ, last weekend we were finally able to tackle the mammoth task of giving the double height external wall that houses the window a top skim coat of lime putty plaster. Honestly, it was a bit of a bumpy ride (not least due to the working height of 4.5meters), but after a few anxious moments and a little nailbiting, we’re confident all is well and we can now say with a fair amount of certainty that we have completed the internal lime work on the Cottage... other than some inevitable filling of course!
As we draw the season of lime plastering the Cottage to a close, and begin to consider the next steps for our freshly plastered walls, I thought it was about time we had a look at the paints and finishes available… specifically those that are suitable for use on lime plastered walls. This post is prompted not only by our position as we consider decorating schemes, but also by the many questions I’ve had on this subject through Instagram - so if this has been a burning question for your for some time, hopefully this post will be helpful for you.
Curing
Before we talk about paints, we first have to talk about curing. We’ve already briefly discussed the curing process for lime plaster in my earlier post, but I think it would be beneficial to look at this in a little more detail. Lime plaster had a very different drying time, and process of application, to gypsum or concrete plasters or renders. Whilst gypsum plasters react with water to recrystallise, forming a set as the plaster dries over the course of a few hours, lime plaster dries very slowly through a process called carbonation. The process of carbonation happens when slaked lime (also known as putty) absorbs water soluble Carbon Dioxide from the air. For carbonation to take place ‘well’, moisture must be present, and the set must happen slowly - the slower the better in fact! If lime is not able to properly carbonate, and it dries too fast, the plaster may fail, crack or delaminate, which is not a situation you want to be in after so much hard work has gone into your plastering. To minimise the risk of this occurring, it’s really important to take the time to understand how this process works, and to properly ‘care’ for your lime after it has been applied. Think of it a little like those Tamagotchi’s many of us had in the late 90s/early 00s… I’m showing my age now, but basically, if you don’t look after it, it will die… metaphorically of course!
After lime has been applied, and indeed even between coats of lime, it’s important to keep it under controlled environmental conditions to allow it time to properly cure and develop its strength. Each manufacturer of lime will have a slight variance on how best to cure their products, but the generally accepted best practise is to provide a humid environment. For external renders and some internal projects, this can be achieved by suspending damp hessian over the area, and spraying this hessian down once a day for at least two weeks. If hessian isn’t required, i.e. if you are working internally, you can spray the mortar/render directly with a pressure sprayer on a mist setting. Do be careful not to damage your mortar/render by spraying too vigorously though, and try not to oversaturate the lime (to the point where cloudy water runs off) as this will wash the lime from the mortar/render leaving you with sand.
It’s also important that the temperature doesn’t fall below 5 degrees celsius whilst lime products are curing. Below this temperature, the water in the mix may freeze, causing frost damage and causing the mortar/render to fail. It’s best to consider the season in which you’re applying lime products before you start, so you can make sure you’re adequately prepared for the work and for protecting the new lime as it cures. Manufacturers differ on how long they anticipate the curing process to take, so it’s always best to consult them to gain accurate insight for the product you are using. As a general rule, you should allow at least 1 month for curing for every 5mm of depth. That means that a wall plastered to 30mm should ideally be left for around 6 months before painting. It’s a long time isn’t it! That isn’t a hard and fast rule though, and as I say it’s worth consulting your manufacturer for curing times.
Paints
On to the fun part! Once your plaster has cured, you can start to think about painting. Whilst lime plaster doesn’t have to be painted, its likely the finish a lot of us will choose, both for its aesthetic benefits and as an added level of protection for our plaster. However, standard, off the shelf paints that you might find in your local hardware store just aren’t going to be right for use on lime plasters I’m afraid. These emulsions tend to contain vinyls, a form of plastic, which form a film as they dry. If applied to lime plaster, this film creates a partial seal over the surface of the lime plaster, inhibiting its ability to breathe, and trapping moisture inside the walls of your home. This trapped moisture can build up under non-breathable paints, causing bubbling and peeling. After you have just spent so much time, effort, and cost on introducing lime products in your home, the very last thing you want to do is ‘fall at the last hurdle’, and find all your hard work undone in a simple coat of the wrong paint. So, how do we know what paints are suitable for use on lime plasters?
Simply put, lime plasters should be used alongside vapour permeable paints, which allow moisture to pass through them without becoming trapped.
Unfortunately, finding these paints can often prove challenging and confusing. Although I wish it wasn’t the case, due to the lack of regulation and clear definition of the term ‘breathable’ within the context of the paints/finishes industry, some brands use the term ‘breathable’ as a clever marketing tool, and don’t necessarily have any scientific validation to back up this claim. For those paints, ‘breathable’ doesn’t mean what we need it to in the context of suitability for application to lime plasters. For example, in the 80s and 90s Dulux used the term ‘breathable’ to describe their paints as they contained less VOCs than competitors, meaning they gave off less harmful fumes and could be ‘breathed’ without safety concerns. Obviously, this definition isn’t the same as we are looking for - we are using the term to relate to the vapour permeability of paints. When it comes to breathable paints for application to buildings, we’re going to define this as:
“a material (paint) that will allow moisture (water vapour) to permeate, move, or transfer through itself by evaporation.”
Breathable paints allow water to evaporate from a surface (in this instance, lime plaster), without slowing or inhibiting the transfer of moisture. Often the first, and simplest indicator of a breathable paint will be its ingredients list. If the ingredients of a paint are filled with vinyls and chemicals, these are unlikely to be what we would class as a breathable paint, as these chemicals will form a seal over the plaster like the vinyls we discussed earlier. A paint with more natural ingredients, like clays and chalks, are more likely to be breathable. However, ingredients aren’t always clear, and aren’t always readily available for us to see, so we need to look for a clearer measure of breathability. There are a multitude of ways to calculate the breathability of paints, but they can often be confusing as different brands use different measures. These measures are aren’t always comparable without involving some serious maths (which, I’ll be honest, is completely beyond me!). However, there is one measure which is fairly universal, and so that’s the measure I choose to go by and reference when I’m looking for a breathable paint: the Steam Diffusion value. The Steam Diffusion value (SD value) is really the simplest way to calculate the breathability of a paint or finish.
A truly breathable paint has an SD value of 0.01-0.05, which means that moisture has to travel between 1-5cm to be able to escape the paint.
Standard masonry paints have an SD value of 1, which means moisture would have to travel for 1 meter to be able to permeate the surface. If you compare these figures, the benefits of breathable paints become much more evident- a wall painted with an SD value of 1 has to move moisture 100 times further for the moisture to be released than a comparative paint with an SD value of 0.01. When moisture cannot easily escape a wall, it can lead to damp problems and peeling paint. All brands will know their SD value, but not every brand will release it. If the SD value of a paint isn’t clearly displayed either on the tin, website or on a data sheet, the likelihood is that this is not a breathable paint and should be avoided. A truly vapour permeable paint that is suitable for application on lime plaster will usually declare their SD value.
What can be a little confusing is when larger paint brands offer different types of paint, some of which are breathable and suitable for use on lime plasters, but others which are not and are specifically for gypsum or woodwork. This is fairly commonplace in the industry, and can actually be very helpful in settings such as, for example, if you had a wall in which 1 external wall was lime plastered, and the remaining internal walls and ceiling were gypsum plastered, or if you wanted to match woodwork to painted lime plaster. Buying from these companies is great, but you do just need to make sure you’re buying the right type of paint for the material you’re applying it to. Whilst breathable paints are usually fine to use on gypsum walls as well as lime, sometimes you might want a different finish, sheen level, or durability rating than the breathable paint can offer, so in situations like this it’s useful to buy from a company that offers both breathable and standard paint ranges with colours transferable across the ranges. Examples of a few companies that offer this would include (but not be limited to) Little Greene, Edward Bulmer, Fenwick & Tilbrook, and Earthborn.
Whilst these companies, and many others, produce ranges of paint suitable for both lime and gypsum walls, what differentiates them is the type of breathable paint they offer. ‘Breathable paints’ is a fairly cover-all term that encompasses a wide range of different paint types and compositions. We’ll take a closer look at these, the pros and cons of each, and I’ll give a suggestion for what you might want to look out for with each of these products.
Limewash
Limewash is one of the oldest paints we have, and dates as far back as Ancient Egypt. A historic paint, limewash is almost always what would have been originally used in historic properties. In its purest form, limewash is burnt limestone watered down to a milky texture, which when applied binds to the calcium in lime plaster. To create colours, pigments are added to the mix, and in modern limewash paints, additives are included to make the limewash more usable. Casein and linseed oils are two of the more common additions, which make the limewash easier to paint on and increase water repellency. Whilst limewash is a generally very breathable and gives a lovely, traditional finish, it does have its downsides. Limewash is relatively weak, which although beautiful, isn’t very durable, and means that it scuffs very easily and will create dust too. It takes a lot of coats to get an even coverage, so whilst that’s great if you enjoy painting, be prepared to apply up to 10 coats on porous background to get a full, even coverage. I wouldn’t advise choosing limewash as a finish if you have children or pets, as contact with the finished surface can result in the limewash rubbing off. If used externally, limewash usually needs to be repainted every 6 months.
Limewash suppliers: Cornish Lime, Little Greene
Lime paint
Lime paint is very similar to limewash, but with more modern additions. Often lime paint will contain a very small percentage of acrylic in its ingredients, which isn’t enough to impair the breathability of the paint, but just helps to stabilise to reduce dusting and so that fewer coats are required - for example, if you needed 6 coats of limewash, you might need to apply 4 of lime paint.
Lime paint suppliers: Cornish Lime, Graphenstone
Distemper
Distemper is another traditional paint, again similar to Limewash, but historically had animal fats added to it. Many distempers now available on the market have glues added to them to increase their durability, but they remain relatively soft and need lots of touching up.
Distemper suppliers: Little Greene, Rose of Jericho
Clay paint
Clay paints are far more durable than limewashes, lime paints and distempers. They contain clays, which increase their stability, and make them easier to apply than the alternatives we have discussed so far. Clay paints are perhaps the most ‘similar’ to the modern emulsions you will have likely used on gypsum surfaces previously, and usually need 2 to 3 coats for full coverage. They contain very low VOCs, and have a very matt finish, as well as covering around 10m2/litre, and drying in 6-12 hours.
Clay paint suppliers: Fenwick & Tilbrook, Earthborn, Edward Bulmer
Mineral paint
Alright, nerdy renovators of the internet (me included!), this is one you’re going to like! Mineral paints are a whole different ball game when it comes to breathable paints. Originally developed in Germany, they have a lovely history which begins with a visit to Italy for King Ludwig I of Bavaria in the early 1800s. The King was very passionate about the arts, and during his visit he fell in love with Italian fresco paintings, and was determined to bring this technique home to Germany (at that time, Bavaria). On his return, he had fresco paintings done, but found that the harsh climate destroyed the paintings quickly. To remedy this, he called upon Bavarian scientists to develop a paint that would have a similar appearance to lime frescos, but would be far more durable and be able to withstand the colder temperatures. His scientists developed what we now know as ‘Mineral paint’, which uses sand to create sodium silicate - known as ‘waterglass’. This water glass is then melted down to a liquid, pigments are added, and this forms the mineral paint.
The key difference between mineral paint and limewash/lime paint is that when mineral paint soaks in, it forms a chemical bond with the silica (sand) in the render, plaster, mortar, or masonry. Where standard emulsions create a layer which sits on the surface of the background material, mineral paints impregnate the surface, effectively becoming a dye. This chemical bond with the silica means that not only are mineral paints suitable for application to a variety of backgrounds, both internal and external, but they are also very, very, durable. Beeck mineral paints, which is the mineral paint we are using in the Cottage (part of an ongoing AD/partnership), has an A1 scrub resistance, which is the highest rating achievable. The durability of mineral paints also means that you have an extended period of not needing to touch up your paintwork. In areas of severe exposure, Beeck mineral paints applied to an external render have been known to last 15 years without requiring touch up, where as limewash on the same property needed repainting after 6 months.
Mineral paints have excellent coverage, and can be used on both lime and gypsum walls. If needed, they can be overpainted in another paint. They apply like modern emulsions, and the colour can be changed without needing to sand back. They don’t require a mist coat, but a primer can be used if necessary, and the first coat should be watered down slightly.
Mineral paint suppliers: Cornish Lime (stockists of Beeck Mineral Paints)
Prep
With all paints, the finish will only be as good as the surface underneath. If you are painting existing lime plaster, do remember that you cannot negate what is already on the walls, and to enable the plaster to breathe, you will need to remove any preexisting vinyl emulsions or peeling paints ahead of applying a new, breathable paint. Do also remember that although a breathable paint, such as mineral paint, may have an excellent scrub rating, lime plaster in itself isn’t particularly wipeable, and doing so could mean you scrub away the plaster itself. As most breathable paints use only natural pigments, they tend to be available in lighter tones, and the darker colours are more expensive as they contain more pigments.
With all of these paints, just because it is called by a certain name, such as limewash, does not mean it is necessarily breathable, so you should always check for an SD value.
I really hope that’s been a helpful article for you. As always, if you have any questions, pop them below, and I will come back to you asap!
Further reading:
Cornish Lime: Guide to paint stripping
Cornish Lime: Guide to Beeck Mineral Paint
Cornish Lime: Breathable Paints Explained
