Plaster provides an inexpensive surface covering for interior walls, but is easily damaged, especially on corners and angles, and cracks easily if there is any movement in the house structure. It is also affected by damp, which can cause the plaster to crumble away from the masonry behind.
Small cracks in plaster can be filled with patching plaster or a proprietary filler, mixed up to a firm consistency. First, use the edge of your filling knife to rake out any loose material from the crack, and if possible undercut the edges of the crack so that the filler can key in better. Brush out any dust from the crack with an old paintbrush, and then dampen it with water so that the existing plaster does not suck all the moisture out of the filler and cause cracks.
Press the filler into the crack, working at right angles to the crack direction; then draw your filling knife blade along the crack to remove excess filler and leave to harden. Ideally, the filler should be left slightly proud of the surrounding surface, so that it can be sanded off to a flush finish when it has set.
Chipped corners are less easy to repair. For small chips, use a fairly stiff filler mix and match the shape of the corner, smoothing it off with glasspaper once it has hardened. Where the damage is more extensive, you will get better results by cutting away the plaster about 50mm (2in) from the corner and then replastering it in one operation. One way of doing this is to pin a batten to one face of the corner so that its edge is level with the surface of the plaster on the other face. The gap between the batten edge and the existing plaster is then filled with new plaster, applied to the dampened brickwork and finished off flush with the existing plaster by running a batten up over the surface. Score the surface of the new plaster lightly, ready for the finishing skim coat to be applied. Repeat the process on the other face of the corner, holding the batten in place against the already plastered face to prevent damage to the corner.
Once the first coat has dried, the surface is moistened again and a thin skim coat of plaster is applied over both surfaces. This coat is polished with a dampened steel float. You can then smooth over the sharp corner angle with a moistened finger.
If the corner being repaired is particularly prone to damage, it can be reinforced with expanded metal angle beading which is pinned to the masonry, or held in place with dabs of plaster, and then plastered over. In this case you will not need corner battens, since the nosing of the angle beading forms an edge for you to plaster to.
Repairing lath-and-plaster ceilings Lath-and-plaster ceilings fail because the plaster that was forced up between the laths no longer provides a strong enough key to
support the plaster’s weight. You can repair small sagging areas by wedging them back up against the laths and pouring quick-setting plaster over the laths from above — by lifting floorboards above ground-floor ceilings, or from the loft for first-floor ones. Larger areas will have to be pulled down and replaced with plasterboard.
Damage to lath-and-plaster ceilings — from a misplaced foot in the loft, for example — can be repaired by bridging the hole with expanded metal mesh which is then covered with a layer of plaster.
Small holes in plasterboard can be patched with scrim (coarse cloth) and plaster. Begin by cutting away the surface of the board for about 25mm (lin) round the hole, to a depth of 3mm (Vain) or so. Cut a piece of scrim big enough to cover this area, put blobs of plaster around the perimeter of the hole and stretch the scrim over it, bedding it in the plaster blobs. When these have set, skim over the repair with two or three coats of plaster to bring its surface level with that of the surrounding plaster. Polish with a dampened steel float when almost set. A self-adhesive wall repair tape is also available. The glass mesh is stuck smoothly over the prepared area and plastered as previously described.
Larger holes must be repaired with a patch of plasterboard. Cut away the damaged board until you reach the centres of the joists or uprights at each side of it, and complete a rectangular opening in the board. Wedge and nail lengths of timber between the joists or uprights to form a rectangular frame, so that these pieces will support the other two edges of the patch. Cut a piece of plasterboard to fit the hole, and pin it in place in the opening. Use plaster or filler to fill the gaps between the patch and the surrounding plaster, and bed fine scrim or paper tape in the wet plaster around the edges of the patch, applying more plaster over it and feathering out the edges for a neat finish. Finally, sponge a thin slurry of plaster all over the repair, ready for redecorating.
To replaster a large damaged area, cut back to a sound edge and dampen the brickwork. Then float on the first coat of plaster proud of the surrounding surface, and draw a long batten up over the patch to remove the excess, resting it on the surrounding plaster at either side. Score the surface and leave to dry, ready for the final skim coat.
Where the area to be patched is too large to be bridged by a levelling batten, place patches of plaster, as thick as the final plaster should be, at intervals across the area, and use these as reference levels for the batten technique. Again score the first coat, and dampen it before applying the final skim coat.