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 finish, skim, or white coat should never be applied until the earlier coat or coats are thoroughly dry and hard, as it is liable to crack if put on before - quite aside from the possible danger of injuring the first-coat work by the pressure of troweling before it is entirely dry and set. A simple putty coat should carry more sand than when the finish is hardened by the addition of plaster. If plaster is used, the mortar should always be gauged (that is, plaster should be mixed with the putty) after it is placed on the mortar-board. The usual process of gauging consists in making a hollow with the trowel in the midst of the pile of lime putty lying upon the mortarboard. This hollow is filled with water, and the plaster sprinkled upon it, the whole then being mixed rapidly with the trowel and put upon the wall immediately, before the plaster has time to set. The proportion of lime and plaster, while variable, averages probably one-fourth to one-fifth plaster.
The finish is skimmed in a very thin coating that is generally less than one-eighth of an inch in thickness. It is immediately troweled several times, dampened with a wet brush, and thoroughly troweled to smooth up the surface and prevent it from chipping or cracking. The water prevents the steel trowel staining the surface, but the plaster should not be too wet, as it will then blister or peel. The whole surface of the finish coat, whether of putty or hard finish, should finally be brushed over once or twice with a wet brush; while, if a polished (or buffed) surface is required, it may be gained by brushing - without dipping the brush into the water - until a glossy surface is obtained.
Especial care should be taken, in the final coat, to finish all joints smoothly and evenly so that the point of jointure will not be apparent. The ceilings are completed first; then the upper part of the wall; and lastly the bottom portions which can be reached from the floor and thus more carefully finished up to the joint.
HOUSE AT PARKERSBURG, W. VA.
R. C. Spencer, Jr., Architect, Chicago, I11.
SHAKESPEARE'S HOUSE AT STRATFORD-ON-AVON.
Here Shakespeare was Born, April 23, 1564. Building Restored in 1857.
The plasterer generally scaffolds the room with boards at a sufficient height to enable him easily to reach the ceiling overhead without raising his arms too high to work each of the coats evenly.
The plaster is applied on the upper part of the walls from the same scaffolding, and the remainder of the work is completed from the floor. If too much time elapses in joining the coats at this point, the joint is likely to show - which is, of course, not serious unless the walls are to be left untreated. Occasionally two men working along together, one on the scaffolding and one on the floor, finish the walls at the same time.
If the old-fashioned wooden angle-beads are used, the plaster should be neatly cut out from each side, forming a small V-sunk angle that prevents the thin edge running up against the corner-bead from breaking off. As a matter of fact, the use of a metal corner-bead makes a far truer, sharper, and straighter angle, and one that does not afterward tear or break the papering when it is put upon the wall.
Angles in the plaster are generally finished with a wooden paddle.
As the hair is used principally to insure a clinch back of the lath, if plaster is applied on a stone or brick wall, a scratch coat is seldom necessary, and the coat of brown mortar is very often used without hair and of about the composition of brick mason's mortar. If a scratch coat is used under these conditions, it is generally mixed with more sand and less hair than when put upon laths.
For a finish where plaster mouldings are to be used, or when for any purpose an unusually straight, level, and plumb surface of plaster is required, three-coat work, put on in the old-fashioned manner, should be demanded. This is necessary in order to get a surface sufficiently level and true to run plaster mouldings evenly, and to avoid the inequalities that are almost certain to occur in all two-coat plastering.
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, bring 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.