How to patch plaster walls

1. Getting the lead out

Before you do any plaster patching, rid the rooms of any residual lead. If you try to vacuum up chips or dust, you must use a special HEPA filter-equipped vacuum cleaner. Some tool rental stores rent these. Obtain all approved cleaning procedures and cleaning products from the EPA's website: epa.gov/lead.

2. Practice patching

You can successfully patch plaster if you have patience and a fair amount of hand-eye coordination. To obtain professional results, you will need to practice and use the correct products. Many old plaster patching jobs fail because people use the wrong materials for the job.

3. The right stuff

Do not use the common drywall joint compounds you often find in home centers. These products are meant to be used only with paper-faced drywall. They bond poorly to traditional plaster. Plaster patching must be done with patching plaster and sometimes plaster of Paris for the best results.

4. Setting compounds

You can achieve moderate success with setting-type joint compounds, and these might be a good product to use to practice your repair techniques. Setting-type compounds are powders that mix with water. The water starts a chemical reaction that causes invisible crystals to begin growing. These crystals interlock within the coarse open plaster to create a strong bond. The advantage to using this material is that it often is sandable. If you make a finishing mistake, you can sand down any excess material. Traditional patching plaster and plaster of Paris are not sandable.

5. Applying smarts

Before applying any product to the walls, they must be clean and free of dirt, grime or grease. All loose plaster must be removed, and all cracks should be enlarged so the top of the crack is at least 1/4 inch wide. If possible, try to make the bottom of the cracks wider than the top. This will take extra work, but the patching compound will interlock with the old plaster like a dovetail joint once it hardens. Slightly dampen the areas to be patched with water just before applying any patching compound.

6. From small to big

The trick to patching plaster is to start with small holes that are no bigger than two inches in diameter. If the hole is deeper than 1/2 inch, then fill the hole only halfway with patching material. The patching material may harden within several hours, and then you can mix new material to finish the job.

7. Tools of the trade

Use regular drywall finish tools to work with the patching materials. Different knives are available. These tools, held at a 30-degree angle to the wall, allow you to spread the patching compounds much like you spread icing on a cake.

8. Finishing up

Both the setting type compounds and the patching plasters can be finished with a sponge as they harden. Professional plasterers often will use a small amount of water on their trowels to make the setting plaster slick as a piece of glass. It takes great skill to perfect this method. You can try to do this as the compounds set, but you have to time it just right.

9. Smoothing it out

The best way to practice is to mix some of the patching plaster and use it to fill a small hole that might be hidden behind a piece of furniture once the remodeling is complete. Use your drywall knives to apply the compound and get it as smooth as possible with little or no excess on the wall. Make sure you use a knife with a blade that is longer than the hole is wide. With the blade spanning the hole, make repeated light strokes over the wet patching compound until no excess material collects on the knife blade. If you press too hard, you might create a slight depression in the patched area.

10. Sponging savvy

Use the sponge to wipe any excess patching compound off the adjacent, sound plaster. As the patching compound gets quite hard, gently stroke it with a damp sponge to make it smooth.