The method of finishing the wall varies somewhat in different parts of the country, and also with the kind of surface desired. In some localities, particularly in small villages, when the walls are to be papered, no finishing coat is applied, but the brown or scratch coat is smoothly troweled. This reduces the expense but a trifle and is not to be recommended, as the walls cannot be as well straightened and the roughness of the plaster will show through the paper.
Skim Coat. - In many of the Eastern States the finishing coat is called the skim coat, and is made of lime putty and a fine white sand - generally washed beach sand. The lime is slaked and run through a sieve into a tight box, and there allowed to stand until it becomes of the consistency of putty, when it is taken out and the sand mixed with it. The box containing the putty should be kept covered to keep out dust and dirt, and the putty should not be used until at least a week old.
The skim coat is put on with a trowel, floated down, and then gone over with a brush and small trowel until the surface becomes hard and polished. In the author's opinion this makes a much better finish than the ordinary white coat, although it is claimed that the latter is better for walls that are to be painted.
White Coat. - (This term is generally used to designate the finishing coat when plaster of Paris is mixed with the lime putty.) In most portions of the United States it appears to be the custom to finish the walls with a thin coat of lime putty, plaster of Paris and marble dust. This makes a whiter wall than the skim coat, and if marble dust is used and the work is well troweled it will take a good polish. Without the marble dust it will not be as hard nor take a polish. For this work the lime is slaked and permitted to form a putty, as with the skim coat. The plaster and marble dust should not be mixed with the putty until a few moments before using, and then only as much should be prepared as can be used up at once, for if left to stand any length of time it will "set" and become useless. It should be finished by brushing down with a wet brush and immediately going over it with a trowel. The more it is troweled the harder it will become. In estimating the quantity of materials required for the white coat, 90 pounds of lime, 50 pounds of plaster and 50 pounds of marble dust should be allowed to 100 square yards.
Sand Finish. - When a rough finish is desired for fresco work, as in churches, halls, etc., the third coat is mixed with lime putty and sand as for skim coat, except that coarser sand and a greater quantity of it is used. Sometimes a small quantity of plaster of Paris is also mixed with it. Sand finish should be applied before the brown coat is quite dry, and should be floated with either clear, soft pine or cork-faced floats. The roughness of the surface desired may be conveniently designated by comparing it with the different grades of sand paper.
Sometimes the brown coat is floated to give an imitation of sand finish, but it is impossible to get an even and uniform surface without using a separate coat. Sand finish is often ruled off and jointed to imitate stone ashlar.