What is the term for the base coat applied directly to the plaster base in a three coat plaster system?

In a three-coat plaster system, the proper term for the base coat that is applied directly to the plaster base is the scratch coat. This layer is crucial as it serves multiple purposes; it provides a rough surface that allows for better adhesion of the subsequent layers. The scratch coat is typically applied first after the base coat, and as its name suggests, it is often scratched or scored to create a textured surface for the next layer to bond effectively.

The sequence typically follows with the brown coat being applied on top of the scratch coat, which is smoother and helps to level the surface before applying the finish coat. Understanding this layer not only aids in proper application but also ensures the durability of the plaster system, as each coat builds upon the previous one, guiding the proper overall finish.