Casein paint uses a milk-based binding agent that is water-soluble. Because it dries to an even, velvety matte finish, it is highly suited for reproduction and was very popular among commercial artists, especially before the invention of acrylics. It may be applied to almost any rigid, non-oily surface such as canvas panel, illustration board, heavy watercolor paper, plaster, metal, wood, masonite, or canvas mounted on panel.
A forgiving medium, casein can be reworked liberally by simply rubbing it with a damp cloth, wet brush, or even an eraser. A dry casein painting can also be buffed to a glossy finish with a soft cloth, giving it an appearance similar to that of an oil painting. Casein paintings have proven to be very archival especially on rigid supports, but beware of applying heavily to flexible supports, like stretched canvas, as it is prone to cracking.