The plastering of a wall generally consists of three coats or thicknesses, each of which is put on in rotation.
Scratch. The first coat to be applied to the lath, tile or brick, or other Coat backing material, is called the first, or scratch coat. It derives this name from the fact that its surface is scratched with a rough brush or comb before it finally hardens, so that the next coat, termed the brown coat, will have a complete bond.
The scratch coat, when put on wood or metal lath, is pressed hard enough so that the plaster goes through the opening in the lath and just covers it, forming a clinch around the lath. This clinch is known as the “key.” On brick, tile or plasterboard, as explained before, the scratch coat cements to the surface and does not require a key as when plastering upon wood or metal lath. Brown The next coat to be applied is the second, or brown coat.
It is Coat termed the brown coat because it generally contains more sand than the scratch coat and so is of a brownish hue. It is used to fill the space between the scratch coat and the face of the grounds and to straighten the walls in such a manner that the third, or white coat may be applied as thin as possible. The scratch and brown coats are commonly known as base coats. The third coat, known as the finish, or white coat, is then applied. White This third, or finish coat, is very thin and for this reason it is often called the skim coat.
Practically throughout the entire United States the white, or third coat consists of a putty made from either lump lime or from a finishing hydrated lime. To this putty the plasterer generally adds, after the putty is placed on the mortar board, about 25% of calcined plaster and also sprinkles in a little sand. Calcined plaster is added so as to counter- act the shrinking of the lime in drying, which would otherwise appear in the form of small cracks, called checking.
Checking will also occur if the plasterer does not wet and trowel the white coat thoroughly when applying. This shrinking can also be overcome by putting in a very large percentage of sand, making it of about the same consistency as the scratch and brown coats. In certain parts of the United States this heavy sanding of the white coat is practiced, but it is a great exception and not so satisfactory as the addition of calcined plaster.
Sand Float Heavy sanding of the white coat is similar to what is Finish known as sand finish, which is more commonly used, and consists of lime putty and sand, to which is added the usual amount of calcined plaster. The sand finish is applied in the same manner as the white coat, except that the surface is floated with a wooden float, giving a rough, sandy effect. A sand finish is generally tinted, while a white coat finish is more often papered.