Casein can be used to provide additional moisture protection over an earthen plaster. Dilute half a gallon of casein binder with another half-gallon of water, bringing the total volume up to one gallon. The binder needs to be diluted to a consistency that will be thin enough, when brushed onto the wall, to soak in and dry without leaving a visibly shiny surface. Do small test patches on your wall or, preferably, a sample board, to determine whether more water will need to be added. In some cases, a full gallon (or more) of water may need to be added to achieve the desired results.
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