If the substrate is weak, some sort of stabilization is necessary. Assuming that you have a crack in stable plaster, we find this method very successful. First remove the plaster from the crack by digging it out down to the wood or other substrate material with a sharp utility knife.
Have lots of extra blades on hand for this. Following the crack, create a V-notch by removing the plaster along both sides of the crack, cutting at an angle until you see the substrate.
This angle will give your infill material maximum bonding surface without sacrificing too much plaster around the crack. You will also be minimizing the amount of debris that you might push into the lath and behind the plaster, causing more damage. If the plaster around this area is delaminated, you can complete any reattachment at this point. Next, vacuum out the crack, then carefully remove additional debris and dust with a small hand broom or paintbrush.
Use a spray bottle and damp sponge to clean the dust off the plaster and prepare the surfaces for infilling with 1 a setting-type compound such as Durabond or 2 plaster such as gauging or plaster-of-Paris. Durabond is a setting joint compound with plasterlike qualities, a built-in bonding agent, and a range of available setting rates—45 or 90 minutes are most common.
Durabond also comes in sandable or nonsandable versions. Now as I stare at the cracks in the walls and ceiling in my home office if you've never lived with one, writers will do nearly anything to avoid writing! I realize I've got more work to do. First, you should know that plaster cracks are a normal occurrence, and you'll probably have to patch a few of them each time you repaint your walls and ceilings. The cracks occur because plaster is fairly brittle while buildings--despite their seeming permanence--tend to move around quite a bit.
Chances are that some of the cracks in your walls run out at odd angles from the corners of windows and doors. These are caused by the fact that your house like most if not all houses has settled over the years. Other wall and ceiling cracks probably run in more-or-less straight lines--almost as if someone had planned them that way.
These cracks are usually caused by seasonal changes in humidity, which creates stress points in the plaster as the framing lumber in your walls expands or contracts as it takes on or gives up moisture from the air. Given that cracks will reappear sometime down the road, your goal with this job is to delay that day for as long as possible. To do so, you'll first need to tape the cracks before repairing them. Otherwise they'll reappear within a few months, or sometimes even a few days.
In the case of stud walls and ceilings, timber laths are fixed between the studs or joists and the plaster is pushed through the gaps between the laths. Rotten laths can result in plaster sagging or bowing. Some cracks and bulges in plaster may be superficial or localised; others result from failure of the masonry or timber beneath, or may indicate wider structural problems within the building. Major repairs underway to a lath and plaster wall; plastering laths on the ceiling is usually fairly straightforward.
Where an area of plaster has become detached from masonry it can sound hollow when tapped. If it has lost its key to laths, it may feel springy.
In both cases the plaster might remain intact as haired lime plaster is generally strong and acts as a sheet, even if areas have parted company with the wall itself.
Conservators use specialist methods to reattach loose plaster or delaminating layers, especially where of historic value, so if in doubt bring in an expert. Problems with ceilings frequently result from the plaster bond having failed, structural movement, failure of joists, damp problems and rotten or beetle-infested laths.
Wallpapers, paints and other finishes can spoil the appearance of plaster surfaces and may trap moisture. When removing, proceed with care and be aware that wallpaper may be acting as a support to defective plaster underneath. Older homes often have walls made of lath and plaster. While it may take a little more skill and time than repairing drywall does, following the simple instructions below should help you do the job easily.
Cover the HVAC vents in the room with plastic sheeting and tape as well to make sure no dust gets into them. Next, use a putty knife to remove any loose plaster pieces and smoothen jagged edges around the area of the crack. Then, wipe away any dust and debris using a rag. Spray some water and dampen the area that needs to be repaired. Mix the dry plaster and water in a large bucket, making sure to follow the instructions given by the plaster manufacturer.
Once it is mixed, pour it from the bucket into a mud pan. Be careful, because you will want the new layer to be even with the rest of the wall. Most cracks will not need more than a single layer of plaster and will need around 24 hours to dry. Use a piece of fine-grit sandpaper and work over the area you patched until it is smooth. Use a damp cloth to wipe away the sanding dust and debris. Then, wipe down the area with a wet sponge and apply a coat of primer.
Once that is dried, you can apply your first coat of paint,. Use a ball-peen hammer and cold chisel to chip off all the damaged plaster from the wall.
Be careful not to chip too hard though, because you do not want to damage the lath behind the plaster. Then, use a utility knife and smoothen the edges inside the hole. You will want to avoid the dry plaster and the lath from absorbing too much moisture. To do this, use a paintbrush to apply a latex bonding agent to the lath and exposed plaster.
Use a drywall knife to fill the hole with plaster. As the first layer of plaster starts to set, cross-scratch or score it using a putty knife. This will help the second layer bind more easily.
Let this dry for 24 hours as well. Apply the first layer of joint compound, making sure the hole is covered and is level with the rest of the wall.
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