PLASTERING Interior plastering is now applied either in two or in three coat ings. Three coats are always necessary on metal or wire lath, the first coat being required to stiffen the body of the material sufficiently to allow thorough working of the remaining coats. Even upon wood laths, three coats make a better job of plastering than two. Extra strength and body are obtained by the addition of the extra coat, pro vided time be allowed to dry out each of the coats thoroughly before the next coating is added. It has now, nevertheless, become the general custom to employ but two coats on the less expensive grades of resi dence work.
The plaster mortar is applied to the walls with a hand trowel of steel, about four and one-half inches wide by twelve inches long, having a wooden handle that is parallel with the back of the blade. After the mortar is put on and roughly smoothed out with the steel trowel, the darby, a long wooden trowel, about four inches wide and three feet in length, is taken by the workman and used—with a scouring motion— to level the plaster surface and work it to an even thickness and uni form density. The flat part of the darby is generally of hard pine, a half-inch or slightly more in thickness.
Three-Coat Work. The best interior plaster work always used to be put on in three coats, and was worked to a final thickness of about seven-eighths of an inch. Of the three coatings, the first is the thickest, so that, when dry, it may be strong enough to resist the pressure of working the coat or coats to follow. A large part of the advantage of three-coat plastering is obtained by thoroughly drying each coat out before applying another, thus securing the added dens ity and strength made possible by forcing the subsequent coating firmly and strongly against the surface upon which it is being placed. Rubbing or troweling up the rough mortar before it finally dries and sets, also makes it much more compact than is possible from working it at the time when it is first applied.
The first coat, called the scratch coat, contains the greatest propor tion of hair, that being useful in strengthening the key or clinch of the plaster behind the edges of the wooden laths, through the crevices between which it has been forced. Before this coat thoroughly dries,
the surface is scratched (hence its name) with a tool designed for that purpose. The surface of the second coat also is sometimes scratched with nails set into a wooden float or darby like that used to rub over the surface, before adding the finish coat. When one coat is entirely dried out before another is applied, this scratching is always necessary, the scratches forming a clinch or tie permitting the subsequent coat to unite the more firmly to the preceding.
The second coat generally contains a larger proportion of sand and much less hair than is necessary in the first coat. The surface of this second coat—or brown coat, as it is called—must be brought up true and even, especially at all angles, and be plumb upon the walls. Before the finishing coat is applied, lumps must be removed and all other imperfections corrected, and the mortar must become sufficiently set to allow the entire surface to be rubbed up with a float or darby and so mad' compact and firm.
To save time, the plasterer adopted the custom of putting his second coat on over the first while the latter was still green. The combined mass (practically one thick coat) was then darbied and treated the same as in two-coat work, over which about the only ad vantage of this method was in providing a rougher sand surface on the second coat than was possible when more hair (always necessary in first coat) was included. Otherwise, substantially the same re sults as are secured by thus working two coats together are obtained in the first coat of ordinary two-coat work, at a saving of both labor and time. While this method does not furnish so good or so perma nent a job of plastering, it is modernly considered as meeting the re quirements of three-coat work, when so specified.