Plaster is a mortar coating that is applied to wall surfaces and ceilings of buildings to make them more airtight and to provide a finished surface. Plastering is putting the plaster on these walls. Certain types of plaster are called stucco. Stucco is often used on the outer walls of houses.
Plaster is commonly made of sand and a cementing agent, such as gypsum, lime, or portland cement. The ingredients are mixed with water. Hair or fiber is mixed with the first and second coats to strengthen the plaster. The hair is goat or cattle hair, and the fiber is Manila, jute, or wood fiber.
Lightweight materials such as perlite or vermiculite may be used instead of sand. These materials absorb sound and are fire resistant.
Plaster bases.
Plaster can be put directly on a masonry wall, but it cannot be put directly on a solid wood wall. The surfaces to which plaster can be applied are called plaster bases. Bases may be of various kinds of building blocks, or brick or stone. Bases may also be made with laths. Laths are metal sheets, pieces of gypsum or fiberboard, or wooden strips that are put on the surface to be plastered to provide a better grip.
Wood laths are laid parallel, with narrow spaces between them. The plaster enters the spaces and forms wedges, called keys. The wedges hold the plaster to the laths. In most modern buildings, gypsum board or metal laths are used. Metal laths are metal sheets about 2 feet (0.6 meter) wide and 8 feet (2.4 meters) long. Open spaces in the sheets allow plaster to penetrate and obtain a firm grip. Gypsum board has a gypsum plaster core between surfaces of heavy paper. The paper and core are pressed together to form a plastering surface.
Plastering.
The plaster is put on the plaster base with a special tool called a trowel. It is smoothed with a tool called a darby and may be made more even with a long straightedge called a rod. Wood or metal strips, called grounds, are placed around openings and along the top of the baseboard as guides for finishing the plastering. If the plastered wall is large, plaster guides called screeds are made on the scratch (first) coat. Three coats of plaster should be used on wood or metal lath. But only two coats are needed on a brick or tile surface.
The surface finish of the plaster may be a white coat of lime putty, which has a thick, puttylike consistency when applied. Gypsum gauging plaster is added to the putty coat to reduce shrinkage and thus avoid cracking. The material is formed into a smooth finish with a steel trowel. The sand-float finish is a rough finish that is made by going over the last coat of gypsum plaster and sand with a special kind of wood or cork trowel, called a float. The sand-float finish looks like rough sandpaper. Special plasters may be applied in different ways to look like natural stones. The most common of these are scagliola, which is an imitation marble; imitation caen stone; and imitation travertine stone.
