When making any repair to plaster, the same type of materials, or similar, to the original construction should be used. Different materials (i.e. lime or cement) have different hardness etc which, if mixed, will lead to a repair failing. The technique for repairing plaster is basically the same whether dealing with plaster walls or ceilings.
Small Area
- Clean out all dust and loose plaster from the damaged area, if the laths are not exposed, cut back the plaster to form a deeper hole – try to undercut the plaster around the sides of the hole so that the new plaster will have something to stick to – try to avoid having tapered edges where the new plaster will have to taper down to almost no thickness on top of the original plaster.
- Mix up some new plaster filler and before applying, dampen the original plaster at the sides and back of the hole using a wet brush or water spray – take care not to over soak the plaster.
- Use a filling knife to fill the hole with plaster filler, take care to push the new plaster into the edges of the hole and any cracks, add enough filler so that it is above the level of the surrounding plaster.
- Use a wet wide bladed filling knife to level off the new filler to the surrounding plaster work – if the filler knife is not wide enough to bridge the repair, use the knife around the edges of it and then work into the middle.
The plaster filler used in the repair may shrink as it hardens, and the surface may pull back from the original plaster level – the deeper the repair, the greater the risk that this will occur. To overcome this shrinkage, a number of layers of filler may need to be applied to build up the surface. Each layer must be allowed to harden before applying the next. Between each layer, the surface of the repair and the surrounding area should be lightly rubbed down using a fine sandpaper – this needs care as lime plaster is extremely soft and easily rubbed away. The surface of the repair filler should be dampened before covering with another layer.
Large repairs
When a large area of plaster needs to be repaired, a more satisfactory finish will be achieved if proper plaster and plastering techniques are used – i.e. apply three coatings, the render, the floating and the setting layers.
- Clear away dust and loose plaster around the hole that will prevent the patch from adhering to the wall.
- Dampen the edge of the hole and fill with patching plaster using a spackling tool. Make sure the patching plaster does not fill the hole level with the existing wall. It should fill the hole to a level just below the surface of the existing plaster – you want to leave enough space for the joint compound to cover the plaster patch and still be even with the wall’s surface. Score the surface of the plaster patch with your spackling tool with vertical and horizontal lines, then allow to dry for 24 hours.
- After the render layer has hardened, dampen the surface and apply the floating layer and bring the level up to about 3mm (1/8 inch) below the surrounding plaster.
- After the floating layer has hardened, dampen the surface and apply the setting layer of plaster and bring the level up to the surrounding plaster.
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