Hey guys! I have recently been doing a lot of testing with boiled linseed oil, lime, shellac, and natural pigments to find the perfect way to finish the walls of my cob house. My requirements were there be nearly no dust off if the wall was touched or bumped. I have kids, so I figured it would be to messy. I wanted the ability to paint or color the mix highly. The house needed to breath through at least some of the coating somewhere in the wall. I also wanted a hardened surface that could with stand some rain and even slight spraying from a hose. Also it needed to be non-toxic.
The just of my problem was box store boiled linseed oil is very toxic due to the nasty chemicals added. The natural stuff is pricey. It also leaves a heavy oil smell behind for some time, and didnt recieve pigment well. I also really didnt like the dust off factor of strait lime and the need to buy refined sand to properly tint the plaster. Also, lime has a very bad manufacturing process.
The outcome is shellac. After the solvent, usually denatured alcohol, off gases the coating is basically food safe. You can make your own strait from flakes, and it mixes to the consistency of house paint with pigments very easy. I wanted to be vary carefull not to completly seal off walls, so I cut down the shellac very thin with Denatured alcohol to the point where I can spray it strait out of a spray bottle and it stops almost all knockdown. The surface is also substantially hardened to the point of grinding off a part of my nail while I was trying to scrub cob off. It still has enough pores to be breathable. If shellac is painted directly on the wall it becomes waterproof and damage proof. The cob feels much more like concrete. The painted surface will probably stand up to light pressure washing. It dries incredibly fast and UV resistant. Ill show some pics of my testing.
As far as pressure washing goes, I also intend to test the ability to paint Shellac directly on wall in shapes and designs and then pressure wash off cob that is not been treated to make a textured surface of the design. This is what the tribal design is inteded for in the second photo.