I've just been trying to research white wash aka. limewash as an exterior preservative for wooden walls. I am hoping to whitewash my batten-board barn this spring but wanted to make triple sure first that it's a good idea!
I found this excellent page with many general details and recipes of how to mix and apply whitewash:
Here is what's listed on the site, but no mention of wood!:
Surfaces suitable for limewash without a primer are:
Lime plaster and renders
Lime and cement based mortars and renders that have proper tooth and porosity
Previously lime washed supports
Masonry supports such as brick, stone, terra cotta (mud) and cement block
Surfaces that are unsuitable for limewash:
Hard troweled renders and plasters
Painted interior surfaces
I've read in some places that whitewash isn't all that suitable for wood because of some pH reaction that causes the wood grain to swell. Other places I've heard it mentioned that it was a traditional practice but without any details. I can see the risk of drying too fast being an issue - although would this be worse than on clay? I will try to apply it whilst temperatures are still fairly low here in Latvia.
Some recipes include linseed oil, flour and salt, and since these are all easily available additives which I can see the sense of using, I'll probably throw these into the mix too.
In fact, I wrote a detailed blog post about it. I was tempted to copy and paste the entire post here, but I see that you are specifically interested in exterior use, whereas mine was formulated for interior. So I'll just give you a link in case you're interested -> 5 Acres & A Dream The Blog: Amish Whitewash.
The challenge for exterior use is that whitewash is water soluble. I did some research on this when I later wrote an eBook. I haven't tried any of these, but here's what I learned.
This is an excerpt from The Little Series of Homestead How-Tos: How To Make Amish Whitewash: Make your own whitewash, paint, and wood stain
How To Make Your Whitewash More Waterproof Hydrated lime is soluble in water, which means it washes off in water. There are several additives you can try to make your whitewash more waterproof.
Linseed oil - 0.5-2% is typically added to decrease its water solubility.
Portland cement - for increased durability, replace up to 10% of the lime with Portland cement type I or II.
Animal fat such as tallow (rendered beef fat) or lard (rendered pork fat) - This method uses quicklime (calcium oxide, see "All About Lime" for an explanation of the different types of lime), so special care must taken to avoid injury during the mixing process. The proportions are roughly 1 part fat, 100 parts quicklime, and 4 times as much water as quicklime. When water is added to the mixture of fat and lime, it causes chemical boiling which melts and binds the fat to the lime.
Here are two of the recipes I found in my research and included in the book. Hopefully, one of them will be useful to you.
Traditional Waterproof Limewash I found this one on YouTube. It relies on the chemical properties of quicklime, so substituting hydrated lime will not work.
50 lbs quicklime
½ lb animal fat (lard, tallow, etc.)
Cut fat into chunks and add to quicklime. Add water, which causes the lime/fat mix to boil. The fat melts and chemically binds to the lime. Mix well. Add more water to keep it from becoming too thick. Can use as a final coat to multiple coats of normal limewash.
A Waterproof Whitewash From an article by M. D. Graham of the Department of Agriculture, Kenya, originally published in the East African Agricultural Journal. It was reprinted in the February 10, 1945 edition of the Advocate, Burnie, Tasmania. I found an online copy at Trove National Library of Australia.
7½ lbs. unslaked lime or lime oxide
1½ lbs. rock salt or cattle salt,
¾ lb. cement,
The method of preparation is: Dissolve the salt in 1 gallon of cold water. When completely dissolved pour on to the lime. Now add to the lime 1½ gal of water, slowly allowing it to slake. Add the cement by sprinkling on a little at a time when the lime is almost slaked. Stir thoroughly. It is advisable to use a trough-like receptacle for the preparation of this lime wash, because terrific heat is genera ted during the slaking of the lime. The use of a narrow-mouthed mixing vessel can very easily result in a mild explosion caused by the steam, with probable serious burns to the operators. It is important that, as soon as the lime has slaked, the wash should be applied immediately and while, still hot. That is why the above proportions have been given, as it has been found that one man can apply this volume before the last of it gets cold. As with plasterwork, this whitewash should be applied, externally, during dull or misty weather, but it will be found that it will set and not flake even if applied during dry, hot weather.
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Note that they both use quicklime (calcium oxide) rather than hydrated lime (calcium hydroxide) which is used in interior whitewash recipes. Quicklime (AKA unslaked or burnt lime) is the most caustic of the lime products.
From experience I can tell you that my interior whitewash worked best on untreated wood. It soaked into the wood and didn't flake off. On pressure-treated wood, it was a different story and I lost most of it eventually from flaking. Traditionally, there are a number of additives to make it more durable: skim milk, alum, glue flakes, or wheat flour. I've not experimented with these, however, because I can no longer find hydrated lime in sufficient quantities locally. So I haven't whitewashed in a while.
Wow, Leigh, thank you so much for a very detailed reply!
I only have hydrated lime here so I will try this first. I have used very simple whitewash on exterior stonework before and although rain will wash it off eventually - I am hoping that the addition of salt and linseed oil will help it adhere nicely.