It is the base coat in a two coat plaster job that would be done on masonry, gypsum lath or other absorptive materials If it's a three coat job, such as on a base of wood or metal lath, it is the second coat, which gets its name from the brown color it turns as it dries The brown coat is the thickest coat applied and gives walls and ceilings their strength If it is not level and smooth, the finish coat will be difficult to smooth
A second layer of plaster that is applied on top of the scratch coat to form a base for the finish coat