Here is a method for producing a durable, weather resistant, and lovely milk paint. The secret is PH. Anionic and Cationic layers will cross link and draw to each other.
Since wood is acid, first you start with the base coating. Alkali will tend to draw in and bond to acid. Then when the alkali coating is almost dry, add the acidic coating. The alkali coating and the acid coating will combine strongly, create fairly stable salts, and cause casine, the protein in milk that is the basis of milk paint, to cross link into a durable finish.
Don’t use whole milk, To much milk fat tends to go a bit rancid. This can leave a long lasting, unclean scent. Take 2% milk right from the carton or use powdered milk mixed just as the recipe calls for. Dissolve as much borax into the milk as it will hold. Just like adding sugar to ice tea, you hit a point where crystals collects at the bottom and won’t dissolve. That means the fluid has as much as it will hold. Add a touch more milk stirring until the last of the borax crystals dissolve. Otherwise the borax crystals can give a gritty feel to the surface. The grit can be nice in it’s own way but is not what I am usually trying for.
This treatment makes a great first layer. borax is a very strong anti fungal, insect preventing, antioxidant layer. It is used in industry for all three reasons. In low levels it is also good for plants, so a bit of leakage is not likely to cause environmental issues. If you want to be serious about preservation, soak the wood.
Here is where some adjustment for effect can be made. The plain borax/milk mix will be slightly yellowish white but mostly clear. If you mix in more powdered milk, you will get a stronger antique yellow look. You can substitute builders or masons lime for borax if you want a more opaque, white appearance. If you go with lime entirely, you make classic white wash. A thin white wash will still allow grain to show through. If you want a chipped paint look, thicken the mix with builders or masons lime. A thin paste will crack when it dries and give a rustic chipped surface. Extra agricultural crushed limestone can give an old paint type bone dry finish. If you want a quality white coverage, add titanium dioxide.
Let the first layer almost dry. Then paint or rub on the second coat. This coat is acidic so it will bond and cross link with the first layer.
Add frozen apple juice to more 2% or powdered milk. As you add it, first the milk will curdle, as you add more apple juice concentrate, the milk will uncurdle. When it is fully uncurldled then you have added enough apple juice.
One of my favorite mixtures is powdered milk diluted by room temperature apple juice concentrate. I keep adding milk until it starts to curdle. Then I add a bit more apple juice concentrate to get rid of the curdle. This makes a hard antique yellow coating.
Take serious heed of this warning, Apple juice while being near harmless internally is a very very strong acid. In particular it is very good at breaking down skin. Many plastic gloves will not last long if you are using apple juice.
Concentrated apple juice varies at about PH 2.3 and It can be much stronger. Even diluted it can be quite caustic to skin if it is ignored. I use heavy duty chemical resistant gloves and a lab apron when working with this paint. If it gets on your skin or gets your clothing wet, clean it or change immediately. I did not, and the injury was quite severe.
If you want a nice green color, add some copper sulfate. This will start out blue but will turn into verdigris (Copper Green) and be totally light stable. Copper sulfate is also acidic, so it goes well in this layer. It also protects strongly from insect damage and is often used agriculturally.
Be careful with the copper sulfate, later it will be fine, but while mixing and applying it is a bad chemical to expose your nose, lungs, eyes, mouth and skin to.
It is also good at this point to mix in some oil. Oil will help bond in the borax, and also help preserve and give a good feel to the wood. Oil will also alter the manner that the casein (the protein in milk that makes it a great paint) cross links and will make the casein more flexible and resilient.
Boiled linseed oil is good, but can grow mildew. Linseed oil will also tend to yellow with time. Tung is superb. Safflower will give the clearest color, but you need the right type of Safflower oil. High Linoleic Acid Safflower oil is a superb drying oil and is used in quality oil paints. Most cooking safflower oil is the other type.
Adding oil will help prevent the borax from leeching out and also help preserve the wood. A bit of oil also gives a good feel to the wood. Oil will alter the manner that the casein cross links and will make the casein more flexible and resilient. Don’t worry that the oil does not mix, stir it well and rub it in. I try to pour just enough oil to just cover the surface of the paint.
After a while the oil may mix into it as an emulsion, otherwise, your rag will tend to pick up both oil and milk. Since I will probably come back later and oil it once more, I keep enough oil on the surface, regularly mixing it in, to cover. You will probably have to add more oil as you go. Without the oil, you get a very dry and flat appearance. Nice in it’s own way but not always what I want.
Too much oil and no milk goes into the mix. You can coat with an oil free coating and then rub in oil before the milk is dry. I just like the feel and convenience of both together. The rag will hold up better with oil in it. I like to wipe it on with a rag and then rub it in like you would polish a car.
I rub in the coating with a square ripped from an old towel. I throw away the old towel after doing this layer, and wash all my equipment immediately after I am done. It is easy to wash immediately but if you wait, it can be very hard to clean up. The towel may have to be replaced part way through your work as the apple juice may destroy it. More oil gives satin finish, no oil gives a chalk flat finish.
Making the first layer light colored, and making the second layer darker but thin, gives an appearance of richness and depth. To color this paint, use pigment powders such as the ones used in ceramics. Apart from the very light colors such as yellow and white, a small amount of pigment will go a very long way.
This is also a quality treatment for leather.
This stuff is the cheapest paint you will ever use. Take some scraps, label them with a sharpie, and make some small batches for experiments. At an agricultural supply 50 lbs of lime will be under $10, while you are there get the copper sulfate for less than a can of good paint. At the grocery store, a jug of milk, a box of powdered milk, a few cans of apple juice, and a box of borax won’t set you back too far. Get this stuff and play with it as you wait for your pigment order to come in. Talk to the folks at the pigment place, they are usually quite a good resource.
There are quite a few examples of milk paint remaining from prehistoric times. In my local downtown, there are still Saloon signs, painted on the old bricks, that can be clearly read from the train tracks, these were made with milk paint. This is a paint that, if done well, can survive drastic change.