This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
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. Two-Coat Work. Most plaster work now consists of only two coats.
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 treatment on the plaster surface. The first coat should run a strong fiveeighths 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 plasterer'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 way as to show through even the finish coat.
Care should be taken to see that each coat invariably is absolutely dry and hard before the addition of another coat is attempted. Otherwise the later coat will fall off, in greater or less part, and it will be quite impossible ever to obtain a good surface finish; while, if it should happen that the first coat is only partially dry when the second is applied, it will be seriously injured by the pressure brought upon it when floating. Its clinch to the lath is thus often partially or wholly broken, sometimes the plaster falling entirely off, leaving the laths exposed.
The finish second coat in two-coat work is the same as the final skim coat in three-coat work.