The second and third coats allow opportunities to obtain a straight and level plaster surface. Individual spots are brought up to an even surface, the plaster then being added and carefully worked between and amongst them, bringing it all to the same face by means of the straight edge. Occasionally it happens that the rough coat is so uneven that some filling in is absolutely necessary to make the wall sufficiently even to receive the last coat. In that case, a mixture of half plaster and half putty may be used in leveling up the rough work.
If no finish coat is to be put on, the surface should be troweled smoothly as the mortar is applied, care being taken to leave no marks, hollows, or uneven places; but if the wall is to be finished or frescoed, it should be left with a floated surface.
Patent Plasters. Patent plasters, such as adamant, etc., are not often employed for private dwellings, being chiefly suitable for mer cantile purposes. The patent plaster has certain advantages that are self-evident—such as quick drying and hardening. Its surface hardens more quickly and resists abrasure longer than the ordinary lime plastering However, a break once occurring, the extreme stiff ness of the mixture makes it liable to extend further and to be of a more serious nature than if the softer, more flexible lime plaster cov ering had been injured in the same manner.
The extra stiffness of most patent plasters is caused by the cement that generally forms an important part of their composition. These plasters are sold ready for use, requiring merely the addition of a sufficient amount of water. They are therefore especially adapted for use by the inexperienced, and are valuable for executing small pieces of work, as they do not present the liabilities to failure, or loss of time and delay, occasioned by mixing up batches of lime mortar.
Back Plastering. Occasionally a wood-framed house is back plastered for warmth. This process consists in nailing a strip of seven-eighths inch furring against the inside of the boarding on each side of the studs. The space between the studding is then lathed (of necessity a slow and bothersome job) and plastered one rough coat of hair mortar, which should be allowed to dry before any lathing is placed over it on the inside face of the studding. As a matter of practice, the efficiency of back plaster is much injured by the fact that the studding, in seasoning after the plaster is set, is likely to shrink away from the plaster, leaving a narrow perpendicular crack on each side of the stud, which permits of the passage of cold air.
Plaster Cracks. Cracks in plaster occur from several causes. If the distance between the ends of the laths, where they join on the studding or furring, is too great, the larger amount of plaster in that place, when drying out, may cause a short crack. Any such spaces should, however, be filled by the lather before plastering is begun. Sometimes, too, especially in the first coats, cracks are caused by the shrinkage or expansion of the wooden laths after the mortar hs s wholly or partially set. The result is a series of narrow cracks parallel to each other and the width of the laths apart. Lath cracks are ordi narily filled in and covered up by later coats, and so do not often appear in the finished plastering. They may, too, be worked out when float ing up the coat before it finally sets. If wide or deep, however, they should be cut out to a width of an inch or so, and filled in with new mortar before adding the last coat.
Cracks of a like appearance are sometimes caused by the rough mortar being too rich, or by draughts of air from open doors or win dows drying out portions of the plastering too quickly. The too rapid drying of plaster with stoves or salamanders, often produces a like result from similar causes. An experienced plasterer should be able to determine the responsible cause and take measures accordingly, using more sand if the mortar is too rich, screening openings to prevent draughts, and using less fire in his drying stoves. In green work, damage already done may be repaired by refloating again before the work becomes too dry, softening the mortar with water if necessary.
Cracks sometimes occur in the angles at the ceiling or corners of the room. When in this location, they may be caused by the shrinkage or settlement of the partition or floor. In the perpendicular angles, especially, they may extend only to the depth of the finishing coats. In that case, the causes are likely to be either too thick plaster, insuffi cient troweling, or an insufficient amount of plaster in the gauged coat—causes which are easily remedied in the remainder of the work.