Plastering can seem a daunting business to the beginner, yet it has only two basic requirements: that the plaster should stick well to its background and that it should be brought to a smooth, flat finish. Good preparation, the careful choice of plaster and use of the right tools should ensure good adhesion, but the ability to achieve the smooth, flat surface will come only after some practice. Most plasterer’s tools are somewhat specialized, but their cost may prove economical in the long term if you are planning several jobs.

Uneven surfaces

Many amateurs tackle plastering with the idea of levelling the surface by rubbing it down when it has set. This approach is very dust-creating and laborious, and invariably produces a poor result. If a power sander is used the dust is unpleasant to work in and permeates other parts of the house, making more work. It is far better to try for a good surface as you put the plaster on, using wide-bladed tools to spread the material evenly. Ridges left by the corners of a trowel or filling knife can be carefully shaved down afterwards with the knife — not with abrasive paper.

When covering a large area with finishing plaster it is not always easy to see if the surface is flat as well as smooth. Look obliquely across the wall or shine a light across it from one side to detect any irregularities.

Crazing

Fine cracks in finished plaster may be due to a sand-and-cement undercoat still drying out and therefore shrinking. Such an undercoat must be fully dry before the plaster goes on, though if the plaster surface is sound the fine cracks can be wallpapered over.

Top coat and undercoat plaster can also crack if made to dry out too fast. Never heat plaster to dry it.

Loss of strength

Gypsum and cement set chemically when mixed with water. If they dry out before the set takes place they do not develop their full strength, and become friable. Should this happen you may have to strip the wall and replaster.

PLASTERING TECHNIQUES

Hold the edge of the hawk below the mortar board and scrape a manageable amount of plaster on to the hawk, using the trowel (1). Take no more than a trowelful to start with.

Tip the hawk towards you and, in one movement, cut away about half of the plaster with the trowel, scraping and lifting it off the hawk and on to the face of the trowel (2).

Hold the loaded trowel horizontally and tilted at an angle to the face of the wall (1). Apply the plaster with a vertical upward stroke, pressing firmly so that plaster is fed to the wall. Flatten the angle of the trowel as you go (2), but never let its whole face come into contact with the plaster as suction can pull it off the wall again.

Build a slight extra thickness of plaster with the trowel, applying it as evenly as possible. Use a rule to level the surface, starting at the bottom of the wall, with the rule held against original plaster or wooden screeds nailed on at either side. Work the rule upwards while moving it from side to side, then lift it carefully away, taking the surplus with it. Fill in any hollows with more plaster from the trowel, then level the surface again. Allow the plaster to stiffen before you smooth it finally with the trowel.

Apply the finishing coat to a gypsum- plaster undercoat as soon as it is set. A cement-based sanded plaster must dry thoroughly, but dampen its surface to adjust suction before finish-plastering. The grey face of plasterboard is finished immediately without wetting.

Apply the finish with a plasterer’s trowel, spreading it evenly no more than 2 to 3mm (Vie to Vain) thick, judging this by eye. Plasterboard requires two coats to build a 5mm (3/rein) thickness.

As the plaster stiffens, brush or lightly spray it with water, then trowel the surface to consolidate it and produce a smooth matt finish. Avoid pressing hard and overworking the surface. Sponge off surplus water.