Mario Baffa, age 12, of Hutchinson, Kan., for his question:

IS THERE A DIFFERENCE BETWEEN PLASTER AND STUCCO?

Plaster is a mortar coating that is applied to the inside wall surfaces and ceilings of buildings to make them more airtight and to provide a finished surface. When plaster is put on outside walls, it is called stucco.

Plasters and stuccos use a mixture made of sand and a cementing agent such as gypsum, lime or portland cement. The elements 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.

With stucco, special additives are included in the mortar to give the finished material an especially hard protective covering. Workers usually apply three coats of stucco, using trowels. Color is added to the final coat. Many finishes can be produced.

Lightweight materials such as perlite or vermiculite may be added instead of sand when mixing the plaster. These materials absorb sound and are fire resistant.

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 also may 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 for the plaster.

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 wide and 8 feet long. Open spaces in the sheets allow plaster to penetrate and obtain a firm grip.

Gypsum board has a gypsum plaster core between heavy paper. It forms a plastering surface.

Plaster is put on the plaster base with a tool called a trowel. It is smoothed with a tool called a "darby" and may be made more smooth and even with a long straight edge called the "rod." Wood or metal strips, called grounds, are placed around openings and along the top of the baseboard as guides for finishing the plastering.

Three coats of plaster are usually used on wood or metal laths. But only two coats are needed on most brick or tile surfaces.

The surface finish of the plaster may be a white coat of lime putty, which has a thick consistency when applied.

Gypsum gauging plaster is added to the putty coat to avoid hair cracking by reducing shrinkage. The material is formed into a smooth finish with a steel trowel.

A "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.