Lime plaster walls are built on top of a support of lathe, which is narrow strips of wood mounted on a frame with gaps between each wooden slat. Lime plaster is layered on the surface of the lathe and plaster is forced in between the slats of the lathe. When dry, the excess plaster binds the plaster to the lathe. Overtime the normal house shifting can cause a separation between the lime plaster and the lathe causing cracks and soft spots in the wall. To repair the wall the lime plaster is reattached to the lathe using plaster repair products.
Use a drill and a masonry bit to drill holes 1 to 2 inches from the crack. When you hit a lathe, use it as a guide line for where to drill. Drill holes on both sides of the crack 3 inches from the hole where you hit the lathe. If you hit a void between the lathe, mark it with a pencil so you will know not to put adhesive in that hole.
Use a vacuum to clean out each drilled hole of loose plaster debris.
Press the spray nozzle against each hole and spray two squirts of wall conditioner into each hole and let it set for 10 minutes. Wipe off any drips from the wall surface with a damp sponge.
Place the plaster repair adhesive nozzle into each hole, using a caulking gun. Squeeze the trigger gently until adhesive begins to flow out of the hole. Do this to all drilled holes skipping the void holes you marked with a pencil.
Drill plastic clamps mounted on drywall screws into existing holes, on each side of the crack, every 8 to 12 inches, until you see adhesive beginning to flow from the holes. Then, stop. Wipe excess adhesive from the wall with a damp sponge. The clamps will draw the plaster and the lathe layers closer together and hold them in place until the adhesive dries.
Let the adhesive dry 24 to 48 hours. Check the label on the adhesive product for drying times. Unscrew the plastic clamps and gently remove them from the wall with a putty knife. Apply all purpose joint compound to all holes and cracks with a 6-inch putty knife. Let the joint compound dry and sand lightly. Wipe the dust off the wall with a clean dry cloth, prime the wall and paint.
A masonry bit works well for plaster repair because it will allow you to drill through the plaster but not the lathe. Plaster repair products come in convenient kits that include the wall conditioner. Ask at your home improvement for suggestions for the best product to use on your cracked wall.
Wear a mask to avoid inhaling plaster dust. Wear safety glasses to protect your eyes from plaster particles and dust.