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The Earth is constantly moving. First, around the sun. Next, around itself. And finally, when the moisture content on its surface changes, soil radically expands and contracts. Therefore, our homes are destined to be heaved to and fro for all eternity. Although it can occur, normally there isn't a great deal of structural damage caused by normal house movement. Most often the movement manifests itself as cracks that appear in the plaster or wallboard surrounding a door or window. The top corner of a window or door is known to be the weakest point in the wall covering.
Plaster and wallboard both react to house movement in the same way, so for simplicity we will use the term wallboard from here on in referring to both.
Working with electricity can be a little tricky; working with gas lines a bit dangerous, but working with cracks in wallboard, at worst, can be time consuming. The process is simple, and if you make a mistake, can be reversed to give you a second or third chance.
The most common error is using joint compound without joint tape. A wallboard crack that moves will crack again the next time the house moves. That's because the joint tape is what gives the repair its strength. There are times when a taped joint will recrack, but only in instances where extreme movement exists.
We don't want to get off the subject, but if properly tape-repaired wallboard cracks continue to reappear, we suggest that you contact a soils engineer to help determine why your home is moving so radically, underground spring, drainage problem, too much irrigation, etc., and then get the proper repair made.
Joint tape and joint compound, a joint-compound tray and a couple of joint-compound knives will be needed to make a proper repair. Spray texture in a can be used if a textured finish is desired.
A tip: The area to be repaired should be completely clean. Although the joint compound will stick to a dirty wall, spider webs and loose particles of dirt and wallboard make it difficult to get a smooth finish. A vacuum works well here. The unwanted particles that make the knife skip and the finish uneven and lumpy will disappear in one or two swipes.
With tools in hand, the first step is to prepare the joint compound. You can purchase a powder and mix it with water or you can purchase a ready mix. We like the ready mix. But even with ready mix, the consistency of the joint compound is sometimes a little too thick and therefore a bit difficult to apply. We find that a consistency somewhere between biscuit dough and pancake batter works best. We usually find ourselves adding a little water to the ready-mix compound making it smoother, lighter and easier to apply. Be careful here though. The joint compound should not be runny or drippy. Also, a too-wet compound will shrink and crack. Not a good thing.
Using a joint compound tray (a long, narrow handheld trough) allows you to evenly cover the entire edge of a 10-inch-wide taping knife with joint compound. The tray is an indispensable tool but can be replaced with a bread pan. Since joint compound is water-based, cleanup is quick and easy, and the pan will be reusable.
With the area clean, wipe a thin layer (about 1/16 of an inch thick and slightly wider than the joint tape) along the entire length of the crack. This first coat is used to hold the joint tape in place. The joint compound can be applied to the tape, but for longer cracks it is easier to apply it to the wall. Lay the tape centered over the crack and use the smaller (4- to 6-inch putty knife) to smooth the tape and remove excess compound. Be sure to hold one end of the tape so that it doesn't slip while you smooth out the tape.
When the first coat is dry - usually about 24 hours - the next one can be applied. We like to use two additional coats, one about 6 inches wide and a third about 10 inches. Imperfections should be sanded out of each coat. Coats should be thinly applied. Thick coats will shrink radically and crack. Also, discard the joint compound as it gets dirty. You will find that the repair is far easier to make when using clean compound.
Caution: Oversanding can tear the paper surface on wallboard. Be careful. A light sanding is all that normally is needed.
The biggest single mistake that most novices make when repairing a wall is to confine the compound to too small a space. The larger the area that is covered with the joint compound, the more evenly it can be feathered into the wall. With the new spray texture in a can, texturing, the hardest part of this kind of repair, is a breeze.