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Once the surface is covered, go back over it, smoothing the plaster to an even thickness. Do not try to achieve perfect smoothness yet (Image 1).
Leave the plaster to dry for another half hour, until it is harder, but still slightly damp (Image 2).
Sweep a clean, dampened trowel blade across the entire surface, smoothing the plaster and redistributing any excess to fill small indents. Hold the blade at a slight angle with only one edge on the plaster to achieve a smooth finish (Image 3).
Leave the plaster to dry for at least half an hour, until the surface is firm enough to touch without moving the plaster, but is still damp. Repeat the smoothing process, again using any excess surface plaster to fill small depressions. If necessary, use a wet brush or garden spray gun to dampen the plaster as you work. Aim for a smooth finish at this final stage; it is more effective than trying to sand rough plaster when dry (Image 4).
Use a small, damp brush to finish edges and corners neatly (Image 5).
To produce a sharp, straight edge when plastering an outside corner on drywall, use a corner bead. This acts as a guide for the finishing plaster, but is covered by the plaster to give a clean finish. Saw the corner bead to the length you need using a junior hacksaw or tin snips (Image 1).
Attach the bead to both walls at the corner ensuring that the bead is tight against the corner (Image 2).
Apply plaster over the top of the corner bead, allowing the plastering trowel to rest on the apex of the bead to give you a good finish (Image 3).
An alternative to smoothing off the first layer of plaster is to apply a second, thinner layer, giving two topcoats. If you want to follow this option, do it after smoothing the plaster for the first time, before allowing it to dry. Most professional plasterers do this to achieve the flattest, highest-quality finish. Once the second layer has been added, continue the step titled Smooth the Plaster from Image 2 onward.
An alternative to using a corner bead on an external corner is to smooth the plaster with a corner trowel. This gives a more rustic finish, rather than a clean, straight line. A corner trowel is not suitable for achieving a high-quality finish; corner beads give better results. A similar finish is achieved on an internal corner by using an internal corner trowel.
Excerpted from Do It Yourself Home Improvement
© Dorling Kindersley Limited 2009