Cracks in plasterwork can appear for a number of reasons, the age of the plaster or the natural swelling and shrinking of the building; or something more serious like subsidence. If cracks keep reappearing or have widened noticeably it may be worth getting them looked at by a professional builder. If the cracks are just due to old age or natural causes they can be filled and painted very quickly. There are two ways of filling cracks; the method you use will depend on how bad they are.
Scrape off any loose or flaking paint along the crack. Follow the manufacturer’s instruction for filling the crack; this will either be application by brush or filling knife.
Leave the filler to dry out. The drying time will be stated on the packaging.
Paint over the filled crack. Try to blend the paint out around the surrounding area, this shouldn’t be a problem if the paint work isn’t too old; if the patch is very visible when the paint has dried you may want to consider painting the whole wall.
Widen crack with the edge of a screwdriver. This will also remove any larger lumps of loose plaster. Scrape off any flaking paint along the line of the crack with a decorators knife.
Brush off any dust. Stick a strip of self adhesive scrim along the crack.
Mix the filler. If you have powdered filler you will need to mix it following the manufacturer’s instructions. If you have premixed filler it will still be a good idea to give it a stir.
Fill the crack by pushing the filler firmly through the scrim with the filling knife. Make sure the scrim is completely covered and feather the edges outward; this will help to blend the repair into the existing wall, try to make the finish as smooth as possible. Allow to dry as per manufacturer’s instructions.
Sand the area with a fine grade sand paper starting at the edges. Do not rub too hard as this will expose the scrim. Wipe off the dust.
Paint over the filled crack. Try to blend the paint out around the surrounding area, this shouldn’t be a problem if the paint work isn’t too old; however if the patch is very visible when the paint has dried you may want to consider painting the whole wall.
When decorating a room it is always a good idea to keep any leftover paint as it will blend better when repairs are needed.
Do not try to sand the filler until it is completely dry.