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Summary: Plaster is a very durable and good-looking wall treatment, but inevitably, it develops cracks of various sizes that are usually fairly easy to repair. In this article you will learn how to repair cracked plaster, from the tiniest hairline cracks to the larger ones that are actually easier to fix than the smaller ones are.
There are many kinds of cracks that can occur in plaster and the various sizes have special ways in which they need to be dealt with, but there is a very basic technique common to repairing all cracks. Do not be tempted to just use your finger and some spackle to repair a crack. It may look good at first, but the cracks tend to show up again after you paint because spackle tends to shrink when it dries.
Safety First
Always be sure to take standard precautionary safety steps when working with plaster, whether it is wet or dry, and make sure you have the following safety items available for your protection: safety glasses, gloves, and a proper dust mask or respirator. Pollen masks or surgical masks are not designed for the fine gypsum dust particles that you will need to protect yourself from.
Basic Technique in a Nutshell
-- The first thing you will need to do is smooth out the surface of the crack by scraping the edges of the opening.
-- After smoothing, use a wide drywall taping broadknife to spread a smooth, thin layer of joint compound along the length of the crack.
-- Depending on the size of the crack, you may or may not need to press paper drywall tape into the compound for the length of the crack.
The following applies for both taped and un-taped cracks:
-- Use taping joint compound if you have a lot of plaster breaks to repair. If you can't find taping compound, you can use multipurpose or all purpose “mud”.
-- Apply a layer of joint compound over the tape that is wider than the underlying layer and feather the outside edges so that the compound extends 2-3 inches beyond the taped area. Let the joint compound dry completely. Lightly sand it down with fine sandpaper to get rid of any bumps or ridges. Don't sand so hard that you dig into the tape or the surrounding drywall. Brush away any loose crumbs.
-- Feather a second layer of joint compound, extending the edges even further out to about 6-8 inches beyond. Let it dry again and sand it lightly again. Brush away any loose crumbs again.
-- Add a third coat that is wider than the second one and takes the feathered edges out to 12 inches. Let it dry and then sand it. Since this is your last chance to get the sanding correct, be very careful to make it smooth.
-- If the original wall has a smooth surface, then smooth out the joint compound and sand it down after it dries.
-- If the original wall has a textured surface, then texture the joint compound, blending it in as much as possible, or purchase a texture spray that matches your current wall texture.
Here are a few ways to deal with the various sizes of cracks:
Superficial Cracks
For a superficial crack, dampen the crack slightly with a plant mister filled with water and patch it with drywall joint compound, using a wide wallboard or putty knife to fill in the gap.
Hairline Cracks
Latex paint will hide hairline cracks in plaster, but the effect is only temporary, so you will need to widen the crack. Yes, I know that sounds crazy. You can use the pointed end of a can opener or a screwdriver to widen it so that you will be able to put a substantial amount of joint compound into the crack. Do not use spackling compound as that tends to shrink and the crack will come back to haunt you.
Deep Cracks
Deeper cracks will require more effort. Check the crack for deteriorating plaster and remove anything that feels spongy or loose. Then thoroughly clean the gap and follow the Basic Technique for un-taped cracks.
Medium Cracks
If the crack becomes wider than a quarter-inch, use ordinary drywall tape to fill it in. The paper tape stabilizes the area and adds strength. Fill the crack with drywall compound, and then anchor the tape into the compound. Press the tape into the wet area, along the length of the crack. Cover the tape with additional compound, feathering the edges of the patch. Try to feather the fill as best as you can for a smooth look. See Basic Techniques for taped cracks.
Large Cracks
Large cracks in plaster walls can be difficult to repair and may have to be cut out and repaired like holes, (which is a whole article in itself) especially if they have been left too long and gotten bigger over time. You need to repair plaster cracks at the first sign of a problem because it is easier to preserve what is already there than to tear it or cut it out. When you start tearing out chunks of plaster, well, one chunk leads to another, and before you know it, your trash bin is full of plaster and you’re out at the hardware store pricing drywall.
Bulges Around Cracks
To repair bulges around cracks, you'll need to chip away at the sagging plaster, again widening the crack. Again, you can use paper tape and wallboard joint compound to fill in the crack. See Basic Techniques for taped cracks.
Multiple Cracks
If you have a lot of cracks to repair in the same area just coat everything with joint compound. This will give you a whole new surface to texture or leave smooth as you choose.
Some Final Tips
-- In some cases, you may want to thin your joint compound a little before you start taping with it, but you don't want to make it too thin. When it has the right consistency it will stick to your knife well and will spread easily, too. As you work with it, you may find that it will begin to stiffen again after a little while, so you may need to stir in more water occasionally. You can experiment with how much water to use, but a nice start would be about two or three tablespoons of water to a pan full of joint compound.
-- Paper tape should be centered and fastened securely over the broken plaster line. If the tape is off target and not centered, your efforts are wasted. Paper tape is better for repairing cracks than fiberglass tape, even though it is a bit trickier to use.
-- Use just enough joint compound under the tape to hold it well. Make sure there are no humps or bumps under the tape. The seam should be as flat as you can make it.
-- Make sure there are no skips in the application of the compound because if the layer is inconsistent or too thin, the tape will not stay tightly glued to the plaster surface in all places. Even tiny skips are no-no's. If the tape covers an area with too little compound on it, you will have blister problems showing up later.
-- If you are taping over raw unpainted plaster, you should apply a coat of drywall sealer before repairs are attempted, at least the section to be repaired. If the repairs are extensive, then put sealer on the whole wall or ceiling.
-- Do all the cracks that run in one direction first. You can't cross a wet seam with another wet seam. When you complete all seams running in one direction, let them dry completely before you do the seams that cross.
-- Finish working with the plaster crack you started with before going on to the next one. Since cracks usually wander all over, you will have a lot of small taped sections running in all directions. Do not overlap the tape where seams come together from different directions. You don't want any extra thickness to cover later.
-- Let the taped cracks dry. With normal heat, this is about 8 to 12 hours, depending on humidity. Keep the room temperature above 50 degrees. A household fan or two set around the job can speed things up.
And that should help you get through the repair of plaster cracks. Now that wasn't so scary, was it? Tedious, perhaps, but not scary any longer.
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