Interior plastering is applied over a base of wood or metal lath for means of holding onto the finish coat of plaster. For best finish results, three successive coats of plaster are applied in most types of work. The first, or scratch coat, is composed of sand and lime, and must be thoroughly compressed into the wooden or metal lath using a trowel (a plastering tool). Before it hardens it is then roughly scratched giving a roughened surface to provide adhesion for the next coat. The second coat, called the brown coat is composed of sand and lime, and is worked to a hard, compact texture, with its surface roughened to receive the final coat. The third, or white finishing coat is composed of plaster of paris, slaked lime (Calcium hydroxide), and white sand, mixed with water to form a paste. It is applied to the wall by troweling, forming a hard, smooth surface. This final coat requires a skilled worker. Moldings, cornices, and relief ornament are cast separately and then mounted into place. In former times ornamental details were molded in their location, from the damp plaster.
McGonagle Plastering and Remodeling 