How-to Repair Plaster

Many people get nervous about learning how to repair plaster. One of the most unsightly things that can happen in a historic home is the cracking of the original plaster. After all, the house has been around a long time, and shifted and settled and the plaster starts looking like a windshield after a run in with a deer. Plaster was the wall covering of choice until the 1950s when gypsum wall board became available. It is hard and durable, however it is not resilient and so it does crack with any settling that occurs. Over a long period of time the plaster can start looking pretty bad. Many people just tear it off and put up wall board, but for those of us who are purists, the wall board just does not have the same look at the plaster, not to mention that the woodwork and trim all has to be redone to look right.

The plaster can often been repaired with spackling compound. Just widen the hairline cracks a bit with a putty knife (one-eighth inch is about right) and then use a blow dryer to blow out the dust. Fill the crack with spackling compound, allow to dry, sand it in a circular motion, and then prime it before painting. That is the easy part of repairing plaster.

Bigger cracks and holes are a little more challenging.

To repair plaster with more damage, you will need to undercut the crack with a putty knife and widen it. Blow out the dust, as with a small crack. Dampen the crack with a sponge and then use your putty knife to fill the crack half full with patching plaster. Allow to firm up, but not dry, and then score the new plaster. Dampen the area again with a sponge and use a broad putty knife to apply the plaster to within one-fourth inch of being flush with the wall. Allow to dry.

Finish with finishing plaster and remove the excess.

Knock out loose plaster. A ball-peen hammer works well for this. Clean out the plaster in the lath to create a surface that the new plaster can adhere to. Brush the area clean of dust and debris, and dampen.

If the hole is small, two inches across or less, but is larger than a nail hole or gouge, fill it with a layer of patching compound, and apply the final coat of finishing plaster. When the plaster is dry sand and prime it. For larger holes, apply a first layer using a six-inch broad knife. Score it when firm and then allow the surface to dry. Moisten, as for a large crack, and then apply another layer of patching compound. Bring this layer within one-eighth inch of the wall surface. Score the patch, and let it dry.

Apply the last coat of finishing plaster, feathering the edges an inch or so beyond the edges of the hole. Smooth the plaster by scraping a straight edge across the wet finish to remove excess plaster. When the patch is completely dry sand and prime it before painting. You can match the texture of the existing plaster by manipulating the finish plaster while it is still wet. Take a good look at the texture of the old plaster. You may need to use a paintbrush, stippling brush, sponge, whish broom or other household item to achieve the texture you want. Experiment a little.

Swirl the plaster in a random pattern, matching the existing pattern as much as possible. To create peaks use a whisk broom. Allow the peaks to begin to stiffen and then smooth over them with a metal float. Always let the plaster dry thoroughly before priming and painting.

If you run into a situation where there is not any lathe or backing behind the plaster to support a new coat you must provide some sort of backing.

After preparing the hole as directed, loop a wire through a piece of rust resistant metal mesh. Roll the edges, insert into the hole, and then flatten it by pulling the wire. Wind it with a stick. Patch the hole, remove the stick and cut the wire for a perfect patch.

Repairing plaster is not difficult and can return your home to its original beauty inexpensively. Just take your time and allow the layers of the plaster to dry completely.