Here’s a little video showing how I make a clay paint to go over a cob wall. It’s a mixture of equal parts, flour paste or methyl cellulose which I get from Gold hill Clay plaster, very fine 70 grit sand and sifted Clay from on site. I added a little bit of Mica to give it a sparkle and a little bit of a Gold hill clay plaster as well to give it a lighter color. I paint this on pretty thick and then use a yogurt lid to smooth it out. This will give it a nice color and help to shed water off the wall. If the structure has a big enough overhang, this could be applied on the outside of a cobin as well as the inside. It can also be sealed with linseed oil and some sort of thinner like mineral spirits to ensure that it will be waterproof.