DO-IT-YOURSELF : Minor Plaster Patches Retain Character of Old Walls

The unmistakable period feel of an old house is often directly related to the beauty and the character of its plaster walls. While the average old-house owner probably should not try to replaster an entire wall or room, minor repair patches can be executed with success by a do-it-yourselfer.

Lath provides a base or reinforcement for plaster. Most old-house owners are probably familiar with wood lath-horizontal boards nailed to the wood framing. When wet plaster is applied, it’s squeezed through these openings and hardened. This in turn forms keys, which are, in essence, mechanical bonds that reinforce or hold the plaster in place.

Today there are different varieties of metal lath available. The original form, an expanded metal sheet that, when pulled apart, forms diamond-shaped spaces for plaster to key is still common, and it’s the one recommended for restoration work. Often referred to as diamond mesh, choose a good 3.4 pound-per-square-yard lath that can be easily cut with a pair of scissors. The other two forms, woven and welded wire, require a paper backing because of their large openings.

The third type of lath, called gypsum board lath, plasterboard or button board, was used in the early 1900s. Essentially a closed gypsum board with a rag paper facing, it was sometimes perforated to provide holes for the plaster to form its important mechanical bond, or key--hence the term button board.

Plaster is commonly applied in two or three coats. The first coat, called the scratch coat, is applied directly to the lath and provides the wall’s base. Generally about three-eighths-inch thick, its roughened or, as the name implies, scratched top provides a surface for the second or brown coat to adhere to. This is also about three-eighths-inch thick.

Essentially a leveling coat, the brown coat provides a base for the one-eighth-inch thick final or finish coat that’s responsible for the smooth whiteness associated with plaster walls. In a two-coat system, such as that commonly used with gypsum board lath, there would be a single base coat followed by the thin, smooth finish layer.

The finish or white coat is a mixture of lime and gypsum. In the proper proportions, it should have the consistency of mayonnaise.