How To... Repair Walls

Wear and tear and slight movement in a building will often damage the plastering, but it is quite straightforward to repair. Put down dust sheets before you start any repair work.

Filling holes and cracks

If a wall is basically in good condition, but has a few holes or hairline cracks, there are different types of wall fillers that can be used to make the surface smooth for decorating. Where an area of plaster is seriously damaged or has come away from the wall (‘blown’), it is better to repair it with plaster. Textured coating, such as ArtexTM, can be used to cover up cracks and uneven surfaces. You can choose from various fillers for minor repairs:

• Interior filler: a general-purpose cellulose filler available ready-mixed in tubs or as a powder for mixing to a stiff paste with water.
• Fine-surface filler: an ultra-smooth fine-particle filler for minor cracks and surface imperfections.
• Deep-repair filler: a ready-mixed lightweight filler for holes up to 20mm deep.
• Fast-setting filler: a filler that sets in 10–20 minutes
• Flexible acrylic fillers: for gaps between plaster and woodwork.
• Foam or expanding filler: for large holes or gaps. Once dry, it needs trimming back with a craft knife.

More extensive repairs

For more extensive repairs there are two main types of plaster that can be used:

• Gypsum plasters: these are the ones used by professional plasterers. They are mixed with water and applied in two coats – an undercoat and a finishing coat. They are quick-setting and generally the most economical, but they are also the most difficult plasters to use.
• General plasters: these are available as two coats – an undercoat or 'repair' plaster, and a finishing coat – either ready-mixed or as powder to mix with water. If a wall is going to be papered, plaster undercoat will give a good enough finish; if it is going to be painted, you need a plaster finish (also known as plaster skim) as well. The finishing plaster is applied before the undercoat is completely dry.

Cracks and holes in plaster

A deep crack can be filled with screwed-up newspaper to make a base for the filler. With larger holes you may need to build up several thin coats of filler rather than one thick one, letting each coat dry before adding the next one. If the entire wall is in a bad condition it may need to be completely replastered – a job for the professionals.

Rake loose plaster from the crack with the corner of a filling knife or scraper. Using a small paintbrush, dampen the crack with water to make the filler dry slowly - this helps it to stop shrinking and falling out of the crack.

Load some filler onto a filling knife and draw it across the hole at right angles to the crack, firmly pressing it in until the filler is just proud of the surface. Leave to dry, then smooth with medium-grade abrasive paper wrapped arund a sanding block.

Repairing holes in plasterboard

Small holes can be repaired in the same way as holes and cracks in plaster, but any larger holes need to have a plasterboard patch fitted into it otherwise the filler will simply fall into the cavity.

Neaten the edge of the damaged area with a craft knife. From an off-cut of plasterboard, cut a patch that will fit through the hole but is just larger than it. Drill through the centre and thread a piece of string through. Tie a nail to the end of the string at the back of the patch (the grey side). Make a loop in the string on the front side and apply filler around the front edge. Keeping hold of the string, push the patch through the hole in the wall.

Pull on the string to manoeuvre the patch into place, with the filler side facing you. Holding it taut, add more filler to the hole. To hold the patch in place while it sets, tie your string to a pencil or timber off-cut and 'wind' it taut against the wall. When the filler is set, cut the string flush with the wall and apply a finishing coat of filler

Lath-and-plaster repairs

As long as the laths (narrow strips of wood that plaster is stuck to) are secure, you can fill small holes in a lath-andplaster wall in the same way as ordinary plaster. Broken laths can often be the reason for plaster crumbling; if this is the case, they need to be reinforced with expanded metal mesh under the new plaster. Application methods vary for finishing plasters, so check the manufacturer’s instructions.

Remove any debris until you get back to an edge where the plaster is still firmly attached to the laths. Using snips, cut a piece of expanded metal mesh the size of the hole.

Position the mesh and secure it in place around the edges with plaster undercoat. (Nailing or stapling on to the laths may cause further damage to the surrounding plaster and lath).

Dampen the laths and apply a plaster undercoat, using a plasterer’s trowel. Force the plaster between the laths to make sure it sticks well. Score the surface of the new plaster in a diamond pattern with the side of the trowel to create a key for the finishing coat. Allow it to set, then apply a finishing coat of plaster.