In multiple coat work, the second coat applied over the scratch coat. In two-coat work, brown coat refers to the double-up basecoat. The brown coat is the coat directly beneath the finish coat.
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.