Painter's undercoat is a preparatory layer applied to a surface before painting. It is usually composed of SEALER and gesso, both of which are used to create a smooth and even surface for the paint to adhere to. SEALER is a liquid or paste that is applied to the surface to prevent the paint from soaking into the material. Gesso is a white, chalky substance that is used to fill in any cracks or crevices in the surface. Together, these two materials form the painter's undercoat, which provides a strong foundation for the paint to adhere to.