People often ask what type of plaster or other materials they need to repair minor cracks in plaster. They are often amazed when I reply drywall tape and drywall compound. You may use setting types or regular old compound. I always use paper tape, just your standard perforated paper tape. The mesh tapes are thicker and harder to hide in a thin coat of mud, are a real bear to cut on clean angles, and I just don't like them.
I started repairing cracks in plaster in the early 1970's and various drywall compounds and a little modified mortar mix are all I have ever used. We will cover the mortar mix part when we discuss replacing a small section of failing plaster all the way to lath.
The process of finishing cracks in plaster is very much like finishing butt joints in drywall; you have to keep it thin and feather it way out to make it look good. The major difference is that they are seldom straight for more than a foot or so and unlike drywall butt joints they sometime cross each other.
When they cross each other you have to work on one, let set or dry before doing the other and where they cross your second tape should not cross the first; remember thin and feathered. You goal is to keep the crack under the center of the tape or as close to that as you can. You may have to start, stop and change angles a few times to accomplish that.
There are 3 types of cracks you may encounter and while all are roughly the same the initial steps are somewhat different.
The smooth hairline crack is by far the easiest to finish. Apply a thin even coat of compound to the wall or ceiling section you are working on, bed your tape into the compound using moderate pressure. If you apply too much pressure it is entirely possible that you will squeeze all the mud from beneath the tape and you will not bond it to the wall. Continue this bedding process until you have done as much of this crack as you can for the session.
The next step can be done immediately or you can wait until the bedded tape has set or dried. Your desires and skill level will determine which method you use. Your objective here is to cover the bedded tape completely with a thin level coat of compound and about 4-6 inches wide; remember thin, level and covered. Sanding in a home you are living in is a major nuisance. If you don't glob it on you won't have to sand it off.
When that is set or dry and any ridges removed you simply come back and feather out the two sides of that top coat about 6 inches on each side. The method is to allow about one inch of the 6 inch drywall knife to ride along the taped portion while the other edge rests on the wall or ceiling. Usually this step will need to be repeated to insure a satisfactory look. Minor sanding or scraping of ridges may need to be done between coats.
The second type of crack is wider and may even have a few loose bits of plaster along the length of its run. The initial step is to remove any loose material, possibly even making the crack a little bigger in the process. Any crack larger or deeper than 1/8" should be skim coated to level and allowed to dry or set before following the exact same process outlined above. I find a simple can open opener, the type that open juice cans, to be about the most effective tool for this job and the other end can open the brew.
The hardest type of crack to repair is a faulted one where one surface is higher or lower than the other. Any change in elevation of more than 1/8" should be investigated as a simple patch may not be enough for it too last. If the crack has loose bits remove then as described above then apply a 4-6" wide layer of compound to the lower of the two sides using the higher side as the riding point for the drywall knife. After it sets or dries proceed with the normal finish steps as described above. You may need to feather the lower edge a little wider than normal to get a smooth look and feel.
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