This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
For 3-coat work, the process of applying and finishing the layers will be described in the order in which they are applied. The coarse stuff is taken in batches from the souring pile, tempered to the proper degree of firmness, shoveled into hods, carried to the rooms, and depos-ited on the mortar board, as at f, Fig. 1. A quantity of mortar is placed on the hawk g, by means of the trowel h, then slices of the mortar are spread firmly and evenly over the surface. of the lathing. The mortar should be tough, hold well together, and soft enough to be pressed between the lath, bulging out behind and forming the key. The thickness of the layer should be fully 1/4in.; in cheap work it is often only a skim coat, and it is not unusual to see the lath through it. After the coat has somewhat hardened, it is scratched over diagonally by wooden comb-like blades, as i, i; from this fact the first layer is often called the scratch coat. The grooves fulfil the same function as the spaces between the lath, to allow a good key for the subsequent layer. The second coat, consisting of fine stuff, to which a little hair is sometimes added, is applied when the scratch coat has become sufficiently firm to resist pressure; the second layer is called the brown or floated coat, because its surface is worked by means of board-shaped trowels, called floats. It is also known as the straightening coat, since all the wall surfaces are straightened and made true. This is effected by first forming a series of plaster bands, called screeds, 5 or 6 in. wide, on the surface to be floated. The surfaces adjacent to the angles are carefully plumbed up from the plaster grounds, but kept about 1/8 in. back from the face, to allow for the finishing coat. Similar screeds are formed along the ceiling angles; these screeds are made straight, and coincide with those at the opposite angles. Intermediate horizontal and vertical screeds are then formed between the screeds adjacent to the ceiling and the plaster grounds; these are usually placed from 4 to 8 ft. apart, and are gauged to line by means of a straightedge. The screeds thus form a system of framing which has been reduced to a true plane; the panels are then filled in flush with the screeds, and firmly rubbed down with a two-handled float, called the derby. The surface is then worked over with a wooden hand float, the coat being firmly compacted by incessant rubbing; when the coat becomes dry during the process, it is moistened with water, applied with a wide brush. A close, firm layer can be obtained only by the thorough, laborious operation of pressing and rubbing the particles of the mortar together. The ceiling surfaces are treated in a similar manner, and the screeds are carefully leveled so as to secure, true and level planes. In order to form a key for the subsequent coat, the surfaces are scratched over with a broom.
Where cornices are desired, they are run before the finishing coat is put on; where the molded surfaces do not project more than 2 in., the body may be of coarse stuff, but where the projection is in excess of this, a cradling of brackets and lath, made to conform to the general profile, should be arranged for its support. Cornice molds are made of galvanized iron, or zinc, attached to a wooden back, which in turn is secured to guide and brace strips. Longitudinal strips are attached to the wall either by nails or a layer of plaster of Paris, and on this the mold guide runs. The coarse stuff is made to conform to the approximate profile with a muffled mold, that is, by forming a layer of plaster of Paris along the edge of the mold, about 1/8 in. in thickness; or an extended profile can be cut out of zinc and attached, temporarily, to the mold. When the coarse stuff has been properly profiled, the surface is coated with gauged stuff and carefully worked over with the correct mold, until an exact and perfect finish is obtained. The internal and external angles cannot be finished by means of the molds, but require to be carefully molded and mitered by hand, using steel plates called jointing tools.
There are several kinds of finishing coats, such as troweled stucco, rough sand finish, hard-finish white coat, etc. In all cases the material is applied to the wall in the form of a stiff paste, by means of a steel trowel, and is spread uniformly over the surface to a thickness of about 1/8 in.
Troweled stucco, consisting of fine stuff and sand, to which a little hair may be added, is thoroughly polished to a glazed finish with a trowel, the surface being kept moist by water applied with a brush.
Hough sand finish can be produced on the stucco by covering the hand float with a piece of carpet or felt, which will cause the sand to raise and present the characteristic sandpaper surface.
Hard-finish white coat consists of gauged stuff, smoothed and polished with the steel trowel; as this material sets rapidly, care must be taken to observe that the second coat is well dried, otherwise the unequal shrinkage will cause hair cracks to occur all over the finishing coat.
A similar finish may be obtained by the use of plasterers' putty, mixed with a small proportion of white sand, and where desired a little white hair may be added; this will give a more durable finish, but it will not set so quickly, and it requires a more thorough working than the former finish.
The space between the plaster grounds and the floor is usually finished with a scratch and a brown coat of plaster, so as to prevent air-currents entering the room from the channels between the furring strips; in cheap work this filling is omitted, the space being covered by the skirting or base,