Slat and plaster walls are a common sight in historic buildings, from small farmhouses to Victorian mansions. Plaster is a traditional finishing material that can be applied over frame, half timber, stone, and brick construction, works well on both flat and curved surfaces, and is easy to clean. While plaster is strong, it's also brittle. That means that many old plaster walls have cracked or broken. Fortunately, repairing an old plaster and lath wall isn't as hard as it looks.
Apply a commercial spackling compound directly to small holes and cracks in the plaster, using a putty knife. Allow the spacking compound to dry. Assess shrinkage--spackle and plaster often shrink and leave a concave spot in one-coat patches.
Apply a second coat if necessary and allow it to dry completely.
Sand the patch. Use a fine-grit sandpaper to smooth the patched area and prepare it for painting.
Remove broken or loose plaster from the area surrounding the hole. Brush away plaster dust, which may prevent the patch from keying into the old plaster. Mix a batch of gypsum plaster according to manufacturer's instructions.
Apply a layer of gypsum plaster to the lath, using a putty knife. Scrape the wet plaster back below the level of the existing plaster. Allow the plaster to set, but do not let it dry fully.
Apply a finish coat of gypsum plaster over the base coat. Scrape the wet plaster to create a level surface that's even with the surrounding wall. Allow the plaster to dry, and sand smooth with a fine-grit sandpaper.
Remove all old plaster from the area around the hole, and renail any loose lath or slats. Spray wooden slats lightly with water to prevent warping when wet plaster is applied.
Attach diamond-mesh metal lath to the wooden slats with tie wires or lath nails. Mix a batch of gypsum plaster according to manufacturer's instructions.
Apply the base coat of plaster over the metal lath, using a putty knife. Start at the sides, then work toward the centre. Allow the base coat to set.
Apply another coat of plaster over the set base coat. Scrape it back to about 1/8 inch below the surface of the plaster, and allow it to set.
Apply plaster over the second coat, and scrape it until it is even with the surrounding wall and completely level. Allow the coat to dry. Sand the finished patch smooth, using fine-grit sandpaper.
It's faster to remove old plaster and replace it with drywall. Repairing plaster in historic buildings is more authentic than replacing it.
Wear a particle respirator when working with dry plaster or when sanding.