Plaster Repair – Cracks & Holes
Plaster Crack Failed Repairs
Most older homes that have cracked plaster are constructed using wood framing materials. In certain climates, these framing members react seasonally to changes in humidity. During warmer more humid months, the framing members absorb moisture from the air and expand. During winter months, the wood dries out and contracts. The houses actually develop stress points or joints where the energy of this movement is released in a visual form that we see. This same movement can also be a result of soils which expand and contract due to changes in moisture.
Repairing these cracks, which move on a regular basis, is somewhat difficult. You must tape over these cracks with a paper or mesh tape. Simply filling the crack with patching plaster will not work. As long as the cracks move even a small amount, the paper tape in most cases will hide the movement of the crack.
Patching the cracks is just like taping a regular drywall joint. You need to feather the joint to hide the tape as in a standard drywall butt joint.
Fixing Plaster Cracks
Cracks are simple to repair. You will need a four, six and 10 inch flexible drywall taping knife and/or broad knife. A drywall mud pan or flat plaster's hawk will be necessary as well. These two things hold the plaster so you can work with it.
Start the repair process by removing all loose plaster. Mix up some joint compound. Mix until all lumps have disappeared. Precut pieces of paper or fiberglass tape to cover the cracks. If the crack is curved (many are!), then you will need several smaller pieces to make the curve.
Apply a 1/8 inch thick layer of joint compound over the crack with your four inch knife. Embed the tape into the wet compound. Drag the knife across the tape and remove half of the compound you applied. If you remove too much, the tape will blister on the second coat! If you leave too much, you will have a giant hump!
Once the compound has set up (becomes hard), you can apply a second coat to hide the tape. I like to use the six inch knife for this job. I also use the 10 inch knife as well. You are trying to add just an additional 1/16th inch over the top of the tape. The larger length knife allows you to taper the compound so the hump is disguised.
When this coat hardens, you can apply the third and final coat. Before you do this, it may pay to scrape off any high spots or use a ceramic tile rubber grout float to smooth out high and low spots. Slightly wet the dried or hardened joint compound and glide the rubber float over the patch. With just the right touch, you will get ultra smooth results.
Patching Plaster Holes
If the lath board is still in place behind the hole, your repair is easy. Simply remove loose plaster and dust, wet the area to be repaired and fill with the setting joint compound. Do not use premixed joint compound for these repairs. These simply do not bond well to the sandy base plaster or wood lath.
If there is nothing behind the hole but air, you need to create a base for the plaster to stick to. There are many ways to do this. Let's say the hole is two inches in diameter. I would cut a one inch wide piece of wood that is six inches long. I would then tie a 16 inch long piece of string around the middle. Apply a liberal amount of construction adhesive to each end of the stick. Smear the glue on the wide face. Insert the stick into the hole so that the glued side faces you. Position the stick so it spans the hole equally. Pull on it with the string so that it comes into contact with the back of the wall that is being repaired. Use the string to hold it in place. The glue should readily stick. After several hours, you should be able to proceed with the patching process.