The saving in this sort of three-coat plastering is made chiefly by the plasterer, in the expense of doing his work. The owner pays mare money than a two-coat job would cost him, and actually receives substantially the same grade of work. The second coat, too, dries more slowly when applied before the first coat is dry and hard, and there is therefore not so much saving in time as is generally believed. If three-coat work is attempted at all, it should be insisted that the first coat be thoroughly dry before the second is added.
The final coat is generally composed of lime putty, with a small proportion of white, clean sand, gauged with plaster of Paris. This gives the whitest finished surface. If a color is considered desirable, a colored sand may be used. All lath cracks or settlement cracks occurring in the previous coats should be cut out and patched before the last coat is applied. The final coat is about one-eighth of an inch thick, and the surface is burnished with the steel trowel to an even and straight surface, and worked sufficiently to free it from chip cracks or other surface defects. The lime for the white finish mortar should be run through a sieve of not less than ten meshes to the inch.
From thus combining the first two coats when green, the next step naturally, in the development of methods of work, was to apply but one coat, making it of increased thickness, and scratching it ready to receive the finish skim or white coat, except when it was desirable to finish the plaster with a rough surface, of. to sand-scour it, as the last process is sometimes called.
The second coat—which should be put on only after the first is thoroughly dry—is substantially the same as the brown coat described above, the rough finish being secured by working the surface of the second coat, before it dries, with a soft-faced float and a mixture of sand with some lime added. Sometimes the surface of the float is of carpet or felt, sometimes of cork or other soft wood. Only so large a surface as may be readily covered at one time, can be floated, darbied, etc., before it has time to set. In this case no hair whatsoever is put in the second coat, as the hair destroys the evenness of the surface that is obtained by the scouring action of the particles of sand rolling around between the surface of the float and the face of the plaster. A long
float is generally used for scouring, and the surface is worked to an even and true face, care being taken not to leave any marks from the instrument itself.
While it is generally the custom to add rough plaster finish on the second coat, in inexpensive work, especially for summer residences, a very artistic effect can be obtained by rough-working the surface of the first coat. If one-coat finish is employed, hair must be used, and the consistency of the coat must remain much the same, whether it is surface-finished or not. In that case, however, it is not possible to work the surface as true and as even as the surface of a second coat.
The brown mortar employed for the first coat should be made of fresh lime used as soon as it is stiff enough to be worked, with strong, well-distributed cattle hair and coarse, clean sand. The first coat of mortar must always be put on with sufficient pressure to force the plaster through between the laths, and so ensure a good clinch. The face of this coat must be made as true and even as possible on surfaces and angles, and plumb on the walls. After the first coat is sufficiently set, it may be worked again with a float consisting of a piece of hard pine about the size of the trowel. Sometimes the face of this float is covered with felt or other material to produce a rough textural treat ment on the plaster surface. The first coat should run a strong five eighths inch in thickness, and should be thoroughly dried out.
It is generally inadvisable to attempt to trowel a two-coat job very smoothly. If the attempt is made to float the first coat when it is too thin or insufficiently set, the instrument is likely to leave marks on the wall, and the plastering is itself likely to crack. It is better to err on the side of caution, as, if the plaster has become slightly too dry, it may easily be dampened by sprinkling water upon it with the plas terer's broad calcimine brush and following it immediately with the float. The use of water in this way has accompanying advantages in that it tends to harden the plastering and to prevent the hairs gathering along the edge of the float, when otherwise they would have to be shaken off every few moments to prevent their rolling under the instrument and being pressed into the surface of the plaster in tufts and rolls, in such a way as to show through even the finish coat.