Scratch coats are the bottom layer or coating of plaster that is applied to a wall. While still wet, this coat is scratched using a trowel or some type of serrated blade. The creation of a web of surface scratches helps the second coat of plaster to adhere to the scratch coat properly by allowing the overcoat of plaster to enter the scratches and create a bond between the two layers.
The application of scratch coating can be utilized with just about any type of wall, including new construction. The scratch coat can be applied to such diverse wall surfaces as stone, sheetrock and even brick. Once this base coat is applied to the wall, it is allowed to dry slightly. However, the creation of the scratches takes place before the first layer of plaster has a chance to set. This leaves behind a rough surface with tiny indentations that can be filled as the second coat of plaster is administered.
Once the scratch coat has been allowed time to set, the finish coat can be applied.