Plastering - a guide for a professional finish

Plastering is an ancient art, still in use today. Plaster is a natural product, hydrated gypsum, with a the chemical formula of CaSO4.2H2O. This dries to the fine white powder which can be mixed with additional water and shaped. It has traditionally been used for interior walls, and to create ornate trims. It is the basis for the painting technique known as fresco where images are painted while the plaster is still wet. The plaster was often stabilized by the addition of materials such as animal hair, or a base such as fabric. Exterior walls may be plastered with powdered limestone (stucco).

Many modern interior walls are made from a pre-formed plaster board also known as drywall or sheet rock. Compared to traditional techniques these are easily installed by nailing to studs or joists, and having the joints and nailheads hidden by plastered tape. Alternatively the entire surface may be covered with a thin coat of plaster, called veneering.

Either way, plastering is still a task which can be difficult for the average do-it-yourself-er. It is a specialized skill and for the best work contractors and homeowners hire a professional for acceptable results. Not only does it require experience to create a smooth surface, but a plasterer is in a constant race with the speed of the chemical reaction as the plaster sets.

However, for small needs, such as to repair wall or ceiling cracks, or small holes, home repairs are accomplished fairly easily. Blown plaster, where the plaster has separated from the backing, is more difficult, but may be tackled by serious handymen.

On a full plastered wall, initial coats of plaster are applied with a skimming float, a large flat-surfaced tool which is held at a slight angle to drag the plaster across a wall. Subsequent coats are added with a technique known as float and set. This simply means that the next coat is applied and floated (smoothed), and allowed to set. To allow each coat to adhere well, there must be a way for it to form a key with the surface beneath. On lath, spaces between the slats allow plaster to ooze between and tightly attach. The next coat will be roughened slightly before it has dried to allow the coat above it to stick tightly, and so on.

For quality plaster work, it is best to call in a professional, except for small repair jobs.

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