In traditionally-built houses made of bricks and blocks, the purpose of plaster is to provide smooth wall surfaces. It is applied to the inner surfaces of the masonry walls that form the outer shell of the building and to both sides of any internal masonry walls. Plastering is a skill acquired only with a lot of practice. Do-it-yourself plasters make the job easier.

USING PLASTER

Most plaster used inside houses is based on lightweight gypsum; cement-based plasters (often referred to as renders) are used for finishing the outside of walls, but may be used for renovation work inside where there is a damp problem. 

THE MAIN TYPES OF PLASTER

Conventional lightweight gypsum plaster is applied in two coats: undercoat around 10 mm (³/₈ in) thick in two layers, and finish coat about 3 mm (¹/₈ in) thick in a single layer. If the surface you are working on is particularly absorbent, choose an HSB (high-suction background) version of the undercoat plaster; similarly, special undercoat plasters are made for dense, less absorbent surfaces. You may also find finish plaster specifically designed for creating a skim coat on plasterboard. 

DO-IT-YOURSELF PLASTERS

The one thing that all "professional" plasters have in common is that they are difficult to apply. They come in large unwieldy bags and have to be mixed with water; they normally set more quickly than the do-it-yourself enthusiast can cope with; it is easy to drag them off the wall whe n you are applying them; you need a lot of practice to obtain a smooth, even finish; and generally they can be put on only in thin layers.

Do-it-yourself plasters have been specifically designed to overcome all these problems. Two grades are available, repair and finish, and are usually available ready-mixed in small tubs, although sometimes as a powder for mixing with water.

Repair or one-coat plaster is much easier to apply than conventional undercoat plaster and can be used in thicknesses up to 50 mm (2 in). It can be employed either for filling deep holes — the kind you might be left with when a waste pipe has been removed from a wall — or to provide a base for the finish plaster, although do-it-yourself repair plasters are often smooth enough on their own as a basis for wall tiles and many wallpapers.

Do-it-yourself finish plaster, also known as plaster skim, can be applied up to 3 mm (¹/₈ in) thick. As well as providing a finish over repair plaster, it can also be used for smoothing a rough surface and for covering over plasterboard.

HOW TO APPLY PLASTER

All types of plaster can be applied with a plasterer's trowel. Carry the plaster to the wall on a hawk, a square flat board mounted on a short handle, and lift a small quantity of plaster off the hawk on to the wall using the plasterer's trowel. Spread it out with a sweeping motion. Always keep the trowel blade at an angle to the wall and allow the plaster to squeeze out through the gap between the bottom blade edge and the wall. Whe n the plaster is starting to dry, it can be smoothed over, holding the trowel at an angle.

To ensure that a plaster repair is flush with the surrounding plaster, use a straight-edged length of wood and smooth over with a side-to-side motion, resting the ends on the nearby dry plaster surface.

Finish plaster is applied in the same way, but is polished with a flat trowel whe n almost dry. Some finishing plasters are applied with a brush, although a plastic spreader will be needed to give it a final smoothing.

PLASTERBOARD

In a timber-framed house, the inner surfaces of exterior walls will be plasterboard (gypsum board), which is solid plaster contained by paper, as will the surfaces of all internal dividing walls. These may be given a skim coat of plaster to cover up the joints between adjacent sheets of plasterboard, or they can have the joints and nail holes filled before painting or papering. Some interior dividing walls in masonry walled houses may comprise plasterboard mounted on timber frames.