One of the conditions for listed building consent was that the colour of the extension must match the yellow ochre limewash of the old building, so the simplest way to do that would be to use the same limewash mix again. Hollins, the architects, instead specified Keim in the listed building consent application, a mineral paint which has been on the market for well over 100 years, and was developed in Bavaria as a more durable substitute for limewash. I’d never heard of it before, but a web search shows it is widely known.
Keim is claimed to last 15-20 years at least, and the company says there are some examples 100 years old. The web site describes it as a liquid silicate paint using a potassium silicate binder with inorganic fillers (feldspar) and natural earth oxide colour pigments. Applied onto a mineral substrate, the binder is absorbed and forms a micro-crystalline silicate structure. This crystalline structure is said to allow the substrate to breathe but stops rain being driven in.
With a much bigger building, now including four gable ends, the idea of a paint that won’t need recoating every few years is appealing, as long as we can get the colour match. (I’m happy to carry on limewashing the old part, for authenticity). So we looked in detail into using Keim.
In the end, we went back to limewash. Here’s why:
The first three free samples from Keim, ordered from their colour chart, were close but not quite close enough. They then offered their free colour matching service if we could send samples. So we carefully chipped off a few square inches from the surface of the old building, wrapped them in plastic and sent them in a parcel to Keim in Telford, receiving a new free sample a few days later. This was a closer colour match to the sample, although its texture is not exactly the same as limewash and may have affected the apparent colour, making it look significantly darker and browner, and slightly less yellow than the limewash on a newly replastered wall on the old building. Keim suggested it might be because we had put the samples on without primer, so kindly sent us a sample of the fixer as well. We tried the paint after leaving the primer to dry for a day, but the colour was still too dark compared with the limewash on the old building. Curiously, when painted onto a completely impermeable surface such as plastic, the paint dried to a pretty exact colour match, but we couldn’t get it to do the same on lime plaster.
Throughout all this, Keim was tremendously helpful and speedy in its response, so I’d have no hesitation in recommending them from that point of view. The paint itself is easy to apply, and behaves on the brush rather like an emulsion. But in the end we went back to our regular limewash supplier, Ingleby of Glemsford, Suffolk.
Firstly, Ingleby charges £80 ex VAT for 20 litres of limewash made with an old Suffolk tallow* recipe to our own colour, Rodgers Flint, making an exact match. A 25 kilogram tub of Keim’s Soldalit to cover the same 60 square metre wall area is £383 including carriage, ex VAT, with extra for the fixative which has to go on first, and there is also the colour match to consider. The fixative for Soldalit is about £55 ex-Vat for a 5 kg tub, so with VAT the total would have been more than £500.
The price premium would surely be well worth paying in many situations because of the durability, which could delay the next big painting job for many years. It would be particularly attractive if an exact colour match is not an issue (eg when the whole building is being painted).
Inglebys reckons that its tallow limewash should be good for at least 5 years and maybe 8 or longer if we don’t mind seeing it fade a bit (which with limewash is attractive). Certainly, their limewash has proved very durable over the last 5 years on our old building, apart from one area where we made the mistake of applying a couple of coats on hot, sunny days. Ingleby’s recipe produces a limewash that covers in only three coats, rather than the minimum of five traditionally recommended (and 8 to 10 according to some purists). Three coats are also required for Keim if the primer coat is included. The amount of work involved is not very different.
So with thanks to Keim for its help and advice, we’ve reverted to a traditional Suffolk paint for the extension.
* Tallow is very water repellent, too much so for some experts in building conservation, some of whom are suspicious even of linseed oil and prefer a simple mix using casein as a binder, because it is more breathable. This seems to be an area of lively discussion among the experts. We started with Ingleby’s tallow mix (called Traditional) on one wall of the old building, then switched to the linseed oil version for the rest, and have switched back to Traditional on the new building. It all looks great. See this link for more on the subject.