Patching Plaster Cracks and Blemishes

First of all clear the crack of dust and loose plaster by scraping with a putty knife or old screwdriver. Where possible, undercut the edges to give better keying. Run a damp brush down the crack; this will help the filler to bond to the wall and, at the same time, clear the crack of dust. Some manufacturers of cellulose filler specifically tell you to omit this procedure, in which case brush the crack with a dry brush.

If the crack is more than 6mm deep, using a steel small tool, fill it with proprietary filler, mixed to manufacturer’s instructions, to within 3mm of the surface. When this has set, damp down and fill to the surface with more fresh filler, flushing it off level with the surrounding surface using either a small tool or steel finishing float.

If gypsum, as opposed to cellulose-based fillers are used, they are liable to shrinkage, so fill the crack proud of the surface and smooth down when it has dried out.

Medium-sized holes in plasterboard

Where the holes are more than 13mm across, use anhydrous plaster and scrim cloth to fill the hole. First, enlarge the area round the hole with a cutting knife, by a further 13mm wide and 6mm deep. Cut a piece of scrim cloth to the size of the enlarged hole. If the hole is very large, two pieces may be overlapped.

Dampen the recessed area round the hole and place small blobs of plaster round the edge. Locate the scrim over the hole. The blobs of plaster will hold the scrim while the entire area is covered with a thin plaster coat. Once the surface begins to set, apply one or two more coats to bring the repair up to the level of the surrounding area. When the plaster coat has almost set, dampen with a brush and polish with a metal float.

Power sockets

Resiting a power socket may necessitate a minor replastering job, using a cellulose filler. Before starting work, switch off the power to the socket and remove the fixing-plate screws. Clean out the cavity and dampen the surrounding plaster before applying the first coat of filler with a pointed trowel.

If there is a gap at skirting level, crumple some chicken wire or use expanded metal to plug the hole, plaster over this, and allow the coat to set. Before applying the second and final coat, dampen the area, screw back the face-plate and polish the final coat with a metal float. Do not reconnect the power until the plaster is quite dry.

Lath-and-plaster repairs

Large holes in lath-and-plaster walls can be repaired, using expanded metal to support the repair area. Hack back loose plaster, undercut, and cut the mesh to size. Either nail it to existing timber uprights or bend it over the laths. Apply plaster with a pointed trowel, a little at a time, building up the surface with thin layers. Leave the first coat for at least 24 hours before damping down for the finish coat. Smooth and polish this with a metal float, damping down with a little water.

Smaller holes in lath and plaster can be repaired with a cellulose filler, provided the laths are intact, to support the new filler. Clean up the area by pulling away torn paper. Where the laths are broken, prepare a plug of paper, of roughly the size of the hole, soaked in plaster. Push this carefully into the hole below the surface, and cover with a thin coat of filler to within 2mm of the surface.

Once dry, dampen and apply a thin finish coat to bring the repair area level with the sound surface. To assist the repaired area to blend in with surrounding surfaces, go over the area with a damp brush to fuse the edges of the repair.

10. November 2011 by admin
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