Perfect Plaster

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Commonly used as a protective finish over exterior walls, plasters also offer durability and beauty for interiors. But choosing the right plaster is not just a matter of aesthetics: The very lifespan of your home can depend on your choices. Today's homeowner has many varieties from which to choose: natural plasters of earth, lime or gypsum, and stuccos of cement or synthetic materials.

A PLASTER PRIMER

Knowing the characteristics of the wide array of plasters in use today, and the pros and cons of each, will help you make the correct choice for your home.

Plasters and stuccos usually consist of three basics: a structural component, a binding agent and some sort of fiber. Sand provides most of the volume of a plaster and serves as the structural component of all plasters and stuccos except gypsum plaster. Depending on the mixture, sand also can lend texture to the plaster.

When water is added to a plaster or stucco mix, binding agents — such as lime, gypsum or cement — cause the sand particles to adhere to each another, creating a pliable, cohesive material that spreads smoothly and bonds onto walls. As the water evaporates, plasters and stuccos set, or cure, and the binding agents create a hard, protective finish.

Fibers frequently are added to increase the plaster's strength, provide reinforcement and reduce or eliminate cracking. Straw, cattails and wool are used in earth plasters; fiberglass is added to cement stucco. In days gone by, horse hair was added to a gypsum plaster base coat.

Plasters and stuccos are spread by hand or trowel, or sprayed on with a mechanical sprayer or pump. Instead of one thick layer, several thin coats are applied to reduce slumping, which may cause plaster to detach from a wall. Thin-layer application also reduces cracking as plasters and stuccos dry. When cured, plasters and stuccos produce a fairly fireproof layer, covering combustible materials in the walls' interiors.

Plaster can provide a smooth or textured finish depending on the plaster and application method. With its small imperfections and ability to be molded around corners and niches, earthen plaster can lend a warm, rustic feel to a home or "create smooth walls with even curves and a polish so fine it feels like glass tossed up from an ocean beach," says Keely Meagan, author of Earth Plasters for Straw Bale Homes. Gypsum plasters and stuccos can take on a multitude of textures as well. Compared to earthen plasters, achieving a uniform finish for gypsum and stuccos generally requires less work.

MOISTURE MANAGEMENT

Controlling the movement of water vapor into and out of a house is one problem that perpetually plagues builders. Not only do builders strive to prevent water vapor from coming in, they also need to find ways to let it out if it gets in.