Drywall mud will adhere to plaster walls, making it simple to patch plaster with a standard joint compound. By taping joints and applying multiple coats of joint compound, you’ll quickly fill holes and patch larger areas. Then, you can use a topping compound to make a smooth finish and even skim coat your plaster to fill in any hairline cracks.
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How Do You Get Joint Compound to Stick to Plaster?
You should have no problem getting all-purpose joint compound to stick to plaster, but some jobs can cause true headaches. If your drywall mud is falling out when you attempt plaster repair, apply a setting-type joint compound first, then use an all-purpose compound for subsequent coats. Setting-type joint compound dries faster and bonds stronger than other varieties. Just make sure to choose an easy-to-sand setting compound.
- Use this setting type joint compound to patch larger areas if standard joint compound fails.
- To increase joint compound bond, thoroughly clean walls to remove grease and dirt prior to patching.
- Avoid glue additives for your joint compound—these make sanding difficult.
Simply cleaning a plaster wall surface with warm, soapy water and a cloth can help to remove grease, dust, and other debris. A clean surface will bond more readily with joint compound. Although glue additives for joint compound are available, they make sanding more difficult. Use them only as a last resort.
How to Patch Plaster with Drywall Mud
To patch a damaged portion of plaster or seal a seam where drywall meets plaster, it’s essential to follow steps that ensure a high-quality patch. Using the proper tools and techniques, you can achieve a seamless repair of your original plaster walls. Here’s how to do it.
Use Paper Tape
If you are patching an area where plaster meets drywall or a damaged area more than 4 inches (10 cm) in size, use paper tape where the edges of the patch meet the existing plaster. The tape will prevent cracks from forming and help the joint compound adhere to the wall. To do this:
- Use this paper tape when patching seams and large areas of damage in plaster.
- Apply a thin layer of joint compound to the wall at the seam.
- Place a piece of paper tape over the joint compound, with the seam bulge facing toward the wall.
- Apply a second layer of joint compound over the tape.
Work carefully to make sure the tape is taut and devoid of air bubbles. Also, ensure that the top layer of the joint compound blends the edges of the paper tape where it meets the wall. This will help ensure a seamless finished product.
Apply Multiple Coats of Joint Compound
When patching plaster, one coat won’t get you the look you desire. Plan to apply 2–3 coats of joint compound. Slowly build up the joint compound to hide the damaged areas of the wall and hide the paper tape you used in Step 1. Properly patching plaster requires patience.
- Apply 2–3 coats of this all-purpose joint compound in order to patch the damage.
- Allow 24 hours of curing time between coats of joint compound.
Allow at least 24 hours for the joint compound to cure between coats. This ensures that the compound has a chance to dry fully. Working over soft joint compound creates warped, rough, and troublesome plaster patches.
Sand Between Coats
It’s important to sand dry joint compound in between coats to ensure a smooth finish. After allowing 24 hours for the compound to cure, sand with 120-grit sandpaper, followed by 150-grit. The 120-grit will remove ridges and bumps left by your putty knife, while the 150-grit will work to smooth the surface of the patch and blend the edges where the joint compound meets plaster.
- Sand dried joint compound after each coat.
- Begin with 120-grit sandpaper to remove ridges and bumps.
- Smooth the surface and edges of the patch with 150-grit sandpaper.
- Applying multiple coats of joint compound without sanding leads to a poor finish.
After sanding, you can apply subsequent coats of joint compound. Just be sure not to skip this step. Loading several coats of joint compound on top of one another without sanding magnifies imperfections. This leads to lumpy, ridged patches that are nearly impossible to sand to a smooth finish.
Apply a Skim Coat
Once your joint compound patches are looking good, it’s time for the final touch. For this step, use a special topping compound to apply a thin layer (or “skim coat”) over the patch. Using this thinner joint compound results in a smooth finish. Make sure to feather the edges of the patched area to blend it with the surrounding plaster.
- After the 2nd or 3rd coat of joint compound, apply a coat of this topping compound.
- Feather the topping compound out beyond the radius of the patch to blend it with the surrounding wall.
- Use the same topping compound to skim coat plaster and repair hairline cracks.
In addition to finishing off patches in plaster walls, a topping compound can be used to skim coat hairline cracks across the entire surface of a wall. Small cracks commonly form in plaster walls over time. You can return your plaster to a like-new appearance with a little topping compound and some practice.
Sand to a Quality Finish
Following your skim coat, sand the plaster patch one last time. Typically, you can begin with 150-grit sandpaper to smooth the area. Then, go over the patch with 220-grit sandpaper. Use this fine sandpaper to create a smooth surface and blend the edges of the patch with the wall.
- Use fine sandpaper to perform a final, fine sanding of the patched area.
- Begin with 150-grit sandpaper to smooth the skim coat.
- Move to 220-grit sandpaper to perfect the finish and blend the edges.
After this process is complete, you’re ready to repaint your wall. When patched in this manner, your drywall compound will stick to the plaster and form a durable repair.
What is the Best Way to Hide Patches in Plaster Walls?
The key to making seamless patches in plaster walls is to take the time to apply a skim coat of topping compound over the patch. This results in a smooth surface that takes primer well and won’t show through the final coat of paint. After patching, use a primer to repaint the whole wall. Primer adheres well to joint compound. When applied with a ⅜” nap roller, it will leave a slight texture across the wall, making the patched area invisible.
- Skim coat your plaster walls with topping compound.
- Prime the entire wall—not just the patched area.
- Finish by repainting the entire wall.
Priming and repainting only the patched area results in an obvious patch job because the paint color will differ slightly from its surroundings. Once you have applied 1–2 coats of primer, finish with 2 coats of colored paint to complete the job. The result will be a seamless, evenly textured wall.
Can You Use Drywall Compound on Plaster Walls?
Drywall compound works excellently for patching plaster walls. For best results, fill large areas of damage with a setting-type joint compound. Follow up with 2–3 coats of joint compound, sanding carefully between coats. Then, finish by skim-coating the patch with a topping compound. After you sand this final coat to a fine finish, your wall is ready for re-priming and painting. By following this process, you will complete a durable plaster patch that won’t show through paint.