Earth Wall Coatings.
Any coating should not be impervious or rigid
This rules out water proofing compounds and cement.
There are many commercial brands of coatings that can cost anything up to an arm, a leg or more.
Mudlark have used very successfully for over thirty years (see website "Little Beach House") a very cheap and simple coating.
Recipe: Mix 2 parts fine clean sand
with 1 part fire clay...from building suppliers
1 part hydrated lime....ditto
Add any mortar ocre to this mix to get the desired colour.. 40cc of yellow will give a deep gold or 10 to 20cc will give a light gold tinge for interior walls. No Ocre will give a soft off white colour depending on the colour of the sand and the fire clay.
Method: Mix water with Bondcrete or Bondcrill at no stronger than 20:1 ie 5%. [b]Any stronger and the coat will form a skin and peel off.[/b]
Rub down the wall to remove any loose or flaking material. Patch any major cracks or holes with a "putty" made from the above soil mix plus no more than 10% cement. (This cement will reduce cracking of the patching material) and enough solution to make a stiff crack filler. (DO NOT USE CEMENT IN RENDER MIX)
Spray all wall with bondcrete/water solution with a garden sprayer till wall is saturated. This mix should soak into the wall up to 20mm depending on the porosity of the soil in the wall. Patch holes and cracks but DO NOT SMEAR OVER SURFACE.
Mix premixed powder with enough bondcrete solution to make a thin paint consistency. About 4 litres of powder to 2-3 litres of solution. Experiment.
One 4 litre can of boncrete should make enough solution to do the entire inside of a large3 bedroom mudbrick house. We have never used 2 full 4litre cans to complete a house inside and out
Paint the mix onto the wall using a full bodied soft hair banister brush. Find one with a "cranked" handle to save your knuckles. Oates is the best one we have found but you may have to buy the dust pan with it.
most cracks up to 3mm will be filled. [b]The final coat should be less than 0.5mm thick.[/b]
Next day gently rub down wall with a dry towel to remove any excess grit.
This mix has lasted well over twenty years on many of our houses and has with stood a fair amount of driving rain. The lime also seems to reduce formation of mould on interior walls.