A mist coat is a watered-down coat of paint that is applied to fresh, dried, new plaster before applying full (non-watered down) coats of paint. New plaster is very permeable, which means it will absorb liquid, including paint. Applying a mist coat helps to seal the plaster and create a bond between the new plaster and the paint. The ratio of paint to water for a mist coat should be pretty thin, almost like a thin soup, to ensure it is absorbed by the new plaster. The exact ratio will depend on the paint you buy, but it is always a good idea to read the tin for guidance. Matt white emulsion is the easy choice for a mist coat, but you can use a colored emulsion that is closer to the top coat for a better match. A mist coat should be left to dry for at least 24 hours before applying the top coat. If you dont apply a mist coat, youre likely to have poor adhesion between your paint and your plaster, and it may crack, flake, or even peel off in sheets in some instances.