Monday, February 12, 2007

Preparing plaster cracks for joint compound

Since I’ve already discussed joint compound and the tools used to apply it, I’m now going to cover how I prepared the cracks in the plaster walls prior to applying joint compound.

If the cracks are large and contain small pieces of loose plaster, you’ll need to remove the old chunks of plaster prior to applying the new joint compound. When I say chunks, I’m merely referring to small pieces about the size of a grain of rice or a single piece of Rice Krispies cereal.

To clean out the plaster crack use the sharp end of a can opener. Since the pointed end is made to withstand opening a tin can, it’s durable enough to dig into plaster. Simply drag the point along the crack to loosen the old plaster and enlarge the crack slightly for the new joint compound. Inevitably you’ll come across cracks where the old plaster chunks, or any loose plaster dust, will not want to come out of the crack. For the new joint compound to work well, you need to remove the loose material. To do so, I use a plastic drinking straw to blow air directly into the crack. Now, keep in mind that when you blow into the crack that there’s a great chance of the debris coming right back into your face... or better yet, your eyes. Consider yourself warned.

Once you have the cracks cleaned you need to determine if you should use any type of drywall joint tape, wall repair fabric, or corner bead to strengthen the repaired crack. Below I cover the three that I used to repair my upstairs bathroom plaster cracks: joint tape, self-adhesive joint fabric, and paper-faced metal corner bead.

Before diving into the explanations below, you may want to read my Drywall and plaster joint compound tools post for a primer on the tools involved with applying joint compound.

Paper joint tape
This is the old standard for covering drywall joints. Simply put down a thin layer of joint compound over the seam with a 4-inch joint knife, place the paper joint tape over the length of the seam, then drag the joint knife over the entire length of the tape to remove any bubbles. After the first coat has dried (which will depend on which type of joint compound you choose), apply another layer of joint compound over the tape with a six-inch taping knife to taper the compound out over the edge of the first coat. When the second coat is dry, gently sand the area smooth. Wipe the dust away and then add another thin coat of joint compound with a 10-inch taping knife.

Best case scenario: you’re finished and ready for priming and painting.

Worst case scenario: more sanding and another layer of joint compound. Just remember, the more joint compound you put on the more sanding you can look forward to. Thin coats work best, and make sure to keep the blade clean of dried pieces of joint compound when applying the second and third layers of mud or you’ll leave scratches in the wet joint compound as you draw the knife along the seam.

Self-adhesive joint fabric
As the name implies, this drywall mesh is easy to apply to the wall because its backing is sticky. Simply cut to length, apply to the wall over the seam (or crack in my case), then add a layer of joint compound with a 4-inch joint knife. Follow the instructions above for the second and third coat. Since this type of joint tape is a little thicker than the paper joint tape, you’ll need to decide when and where to use it. I used it to patch plaster cracks only in areas where there’s no concern over the thickness of the joint (this isn’t a concern when applying to new drywall, because drywall edges are tapered to accept tape and mud). If a crack is mid-wall, I’d recommend the thinner paper joint tape, if any at all. Remember, your repair will need to be discreet so when the wall is painted it won’t stand out.

Paper faced metal corner bead
To add joint compound to corners (both inside and outside corners), drywallers have traditionally used metal corner bead. It’s thin, easy to cut to length, is pre-bent at a 90-degree angle to fit perfectly into corners, and features a slight recess molded into the corner to accept extra joint compound (to assure the metal won’t show through the joint compound in the corner).

More modern alternatives include plastic corner bead, which is easier to cut and isn’t as prone to cutting your hands as its metal counterpart. Another modern alternative, and the choice I went with, is a corner bead that’s a hybrid of paper joint tape and metal corner bead (see the photos to the right). This product is beneficial because the paper is super thin, yet the metal corner is very durable. I used this product to patch cracks in the corners of my plaster walls and at the point where the walls meet the ceiling.
Now that I’ve covered the tools, joint compound, and crack preparation, next time I’ll show you some of the variety of cracks I repaired in my plaster walls.


  1. As a 24 year veteran in the wall industry, I wouldn't use anything but metal corner bead on outside corners. Reason being, the paper bead is extremely flimsy and one bump with a dresser etc... will put a dent into it. The plastic bead requires durabond or sta-smooth to reinforce it's strength and, if not properly nailed off; it will crack when bumped.

  2. well thats metal dents also. plus metal screws into studs and when house settles or expands and contracts, mud cracks cause bead moves with the beams. there are better alternatives now a days.