The most likely time you will use a DIY plaster undercoat (repair plaster) is to repair damage to existing plaster - either damage you have created (perhaps by installing pipes or electric cables) or damage that has happened through accident or old age. It is also ideal for finishing off where you have bricked up a fireplace or a door opening.
The first thing to do (especially with blown plaster) is to lay a dust sheet on the floor and cut back all existing loose plaster until you have a sound surface with solid edges. Use a club hammer and bolster chisel to do this (1) and remove all loose dust and debris with a stiff brush (2).
Unless the plaster undercoat comes ready mixed, use a large clean bucket to mix up the powder with water. Use a clean wooden stick to do the mixing. It is easier if you add the powder to the water rather than the other way round. The plaster should have a thick creamy consistency.
If the repair (or new plasterwork) includes a corner, fit metal angle bead to this first. Not only will it give a neat corner, but the bead will provide a guide for your levelling board. Cut the bead to length (with a hacksaw) and prime the cut ends. Fix it to the corner with dabs of plaster (3). Check it is vertical and leave the plaster to dry.
Before you apply the plaster, dampen the wall surface, using a large paint brush - this will help adhesion and make it easy to 'work' the plaster. On very absorbent surfaces (aerated concrete blocks, for example), add one part PVA adhesive to five parts water.
Transfer an amount of plaster to a hawk - a flat board about 30cm (1ft) square. You can buy a ready-made hawk or make your own from a piece of plywood nailed to a block of wood with a short handle (a bit of broom handle, for example) attached to it.
Carry the hawk to the wall and hold it in front of the area you want to plaster. In one movement, tip the hawk towards you, lift off an amount of plaster with the edge of a steel plasterer's trowel and press the plaster against the wall with an upwards sweep of the trowel (4).
The method of application is crucial - you start with the trowel at quite an angle to the wall and gradually reduce this angle as you move it up the wall. Do not allow the trowel to get flat against the wall or you will pull off the plaster you are trying to apply.
Carry on adding plaster until the whole area to be repaired is covered. Remember always to keep the trowel at an angle to the wall and leave the plaster slightly proud of the surrounding surface.
To get the surface level, use a piece of straight-edged timber held at an angle (or if you have one, a metal rule) moving it up the wall with a side-to-side action, holding it firmly against the existing sound plaster as a guide (5). When plastering a whole wall, timber screeds are attached to the wall (and levelled) before plastering to act as the guides for this straight-edge.
Take care removing the straight-edge or rule from the surface together with the plaster you have scraped off. You should now be able to see any hollows which can be filled with more plaster before ruling off again.
Keep a bucket of cold water handy at all times, so that you can keep tools clean - especially the plasterer's trowel.
Allow the plaster surface to harden (but not completely dry) before dampening it and smoothing it flat with the trowel (still held at a slight angle) (6). If you are applying a finishing coat, scratch the surface of the undercoat with the edge of the trowel to provide a 'key'.
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