The brown coat is applied over the scratch coat to prepare the plaster base for the finish coat application. To start with, some systems are made up of two coats and some are three. Three coat work is for systems that are constructed with lath and two coat work is for direct application to masonry or concrete backup.
The brown coat is applied over the scratch coat to prepare the plaster base for the finish coat application.
The three coats consist of two base coats and one finish coat. The first base coat is called a scratch coat, the second is called a brown coat. In two coat work, there is a single base coat and a finish coat.
The purpose of the first base coat, the scratch coat, is to embed the metal lath and provide a base for the brown coat. The scratch coat gets its name from the fact that it is physically scratched with horizontal marks. These scratches create a key; for the next coat to grab onto and a shelf for moisture to aid in curing of the brown coat. The brown coat covers the first base coat and creates a plane surface, leading to the best possible results for the finish coat. The finish coat is the thinnest of the coats, and its purpose is to impart a decorative surface to the plaster.
Scratch, brown, and finish coats all have slightly different proportions. Each one has a range that is allowed, but all are specified by volume. One reason for this is to give the contractor some leeway in choosing a mix that works best with the contractor’s specific materials. Another reason is that certain properties of the hardened plaster can be accentuated in each coat. The first coat will provide a hard base for the system without a great deal of shrinkage. The greater sand content in the second base coat might generate less shrinkage to create a better base for the finish than the first base coat. The finish coat should be hard to resist abrasion and other surface damage.
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