* In the Eastern States dwellings of moderate cost are generally plastered with two-coat work, the first or scratch coat being brought out nearly to the grounds, and carefully straightened to receive the skim coat.
The scratch coat should always be made "rich," and should contain plenty of hair or fibre, as it forms the foundation for the brown and white coats. This coat is generally put on from 3/16 to ¼ inch thick over the laths, and should be pressed by the trowel with sufficient force to squeeze it between and behind the laths, so as to form a key or "clinch." It is this key which holds the plaster to the laths. When the first coat has commenced to harden (the time varying from two to four days) it should be scored or scratched nearly through its thickness with lines diagonally across each other, about 2 to 3 inches apart. This gives a better hold to the second coat.
The first coat should be thoroughly dry before putting on the second coat, but if the surface is too dry it should be slightly dampened with a sprinkler or brush as the second coat is put on.
A great deal of plastering (sometimes called "green work") is done where the brown coat is applied from the same stage, and as soon as the scratch coat is put on. When done in this way the scratch coat is generally made very rich and the brown coat largely of sand, the brown coat being worked into the scratch coat so that it really makes only one coat.
All intelligent plasterers admit that it makes better work to let the scratch coat get dry before the brown is put on, but as it takes more labor and also more lime to put on the plaster in this way, they will not do it unless it is particularly specified. Besides not giving as good a wall, applying the brown coat to the green scratch coat also causes the laths to swell badly, which, when they dry, causes cracks in the plastering.
The second or "brown" coat is put on from ¼ to 3/8 inch thick. With this coat all the surfaces should be brought to a true plane, the angles made straight, the walls plumb and the ceilings level.
On the walls the plastering can generally be brought to a true plane by means of the grounds, if the latter are set true and the wall is not too large or without openings. On the ceilings, however, there is usually nothing to guide the plasterer in his work, and the consequence is that most ceilings, particularly in domestic work, have a rolling surface, as can be detected at the edges of the ceiling.
Screeds. - The only way of obtaining a true plane on ceilings and on walls, where the grounds are not sufficient, is by screeding, which is done by applying horizontal strips of plaster mortar, 6 to 8 inches wide and from 2 to 4 feet apart, all around the room. These are made to project from the first coat out to the intended face of the second coat, and while soft are made perfectly straight and out of wind with each other by measuring with a plumb, straight-edge, etc. When dry the second coat is put on, filling up the broad horizontal spaces between them, and is readily brought to a perfectly flat surface corresponding with the screeds by long straight-edges extending over their surface.
On lathed work, if the studding or furrings have been properly set, screeding should not be necessary except on ceilings, but on brick or stone walls it is impossible to get true surfaces except by means of grounds or screeds. Screeding was formerly done much more extensively than at present; now it is seldom required except in very expensive buildings. Screeding can be done only in three-coat work. Before the brown coat becomes hard it should be lightly run over with the scratcher to make the third coat adhere better. If part of the walls are to be plastered on brickwork and part on laths, the scratch coat is put only on the laths, and when this is dry the brown coat is spread over the whole, including the brickwork. Brick walls that are to be plastered should have the joints left rough or open, and the walls should be brushed clean of all dust and slightly dampened before putting on the mortar. In very dry weather the brick walls should be sprinkled with a hose just before plastering.