Most of the homes built during the past two or three decades have walks and ceilings made a gypsum board rather than plaster. Often referred to as plasterboard or drywall, this type wall or ceiling is generally less prone to cracking than plaster, but it is not entirely immune to this program.

Cracks do sometimes develop along seams, or near the corners of door and window frames, because of setting or warping of wood structural members inside the wall. Also, drywall surfaces often have problems with popping nails, and they are easily damaged or holed when accidentally struck a heavy blow or when something falls or bangs against the surface.

Fortunately, repairing drywall is fairly easy in most cases, even when large sections have been damaged by water leaks, or when an alternation has left gaping holes that just be filled or covered over.

The simplest problem to cure, and the one most frequently encountered, is that of nails popping out of the surface and pushing off the compound that was originally applied over the nailhead to conceal it. This usually caused by studs inside the wall warping or shrinking so that they pull away from the back of the panel and the nail lets go, but it can also be caused by the use of improper nails, or poor nailing techniques when the panels were originally installed.

To correct this, start by driving the protruding nail back in (remove any compound that is still stuck to the head) so that its head is recessed just below the surface of the wall. If the nail is so loose that it won't hold, remove it entirely. Otherwise drive the head below the surface with a nailist, but try to avoid driving it all the way through the top layer of paper on the gypsum board.

Next, drive another nail in about 1 1/2 inches to 2 inches above or below the original nail (into the same stud or beam), but this time use an annular-threaded or ringed drywall nail, because this type nail holds better than a conventional nail.

Before driving the nail, press the wall panel firmly back so it is in solid contract with the stud, then drive the nailhead flush with the surface. Then recess the head slightly by giving it one more light blow with the hammer. The idea is to hit it just hard enough to create a slight dimple in the paper facing, but not hard enough to break or tear the paper.This leaves a shallow depression that can then be filled in with a suitable patching compound to leave a smooth surface.

For filling the nail holes and the depressions around the nailhead, use either a ready-mixed vinyl spackling compound or a ready-mixed drywall joint cement. Both these materials come in powder form too, and are always of the right consistency. Mixing the powdered type with too little or too much water can result in a poor patch that will shrink and-or crack excessively.

The compound is best applied with a 3-inch-or-4-inch-wide flexible putty knife, smearing the first layer on in one direction and then cross-stroking lightly at right angles to smooth off the excess. In most cases at least two applications will be required to get a smooth finish, but wait until the first layer is hard before the second layer is applied.

To get a really smooth finish wet the blade of the knife by dipping it in water after the final layer has started to stiffen slightly. Then drag the wet blade across the patch while pressing it almost flat against the surface. This should leave a smooth enough finish so that little or no sanding will be required.

To fill cracks in gypsum board, use the same ready-mixed spackling compound, but first cut the cracks out slightly by using a pointed tool, such as a beverage can opener. Undercut the crack so it is slightly wider along the inside than it is at the surface, then brush out loose material and pack with compound as described above - again using two or three applications.

To repair a hole that goes all the way through the gypsum board, leaving no backing against which a patching compound can be applied, there are two methods, depending on the size of the hole.

Small holes, such as those left when a doornob breaks through, can sometimes be filled by first creating an artificial backing in the hole. The easiest way to do this is to shove wads of paper or folded pieces of cardboard into the hole until they become wedged between the panel and the back of the other side of that wall.

Then wet the paper and the edges of the hole. Mix some patching plaster and pack this in round the edges, but at this point make no attempt to cover or fill in the entire hole. Allow this layer to dry hard, then apply more plaster to cover the rest of the exposed paper, filling the hole about half-way to the surface.

Again let this dry hard, then apply a third coat that almost, but not quite, fills the hole entirely. Finish after this hardens by smoothing over with spackling compound as described earlier.

The technique just described works well in most cases where the hole is no more than 2 or 3 inches across. When the hole is larger, or when an entire section of one panel has been badly damaged (by a water leak, for example), the best solution is to cut out the damaged section and replace with a new niece of gypsum board.

To do this, use a sharp utility knife or small keyhole saw to cut the damaged area from one stud to the next on each side of the damaged area. Studs are usually spaced 16 inches apart, center to center, but just cut sideways in each direction until the stud is met. Then use a chisel to remove enough of the damaged piece to expose about half the stud on each side, as shown.

Now cut a new piece of gypsum board the same thinkness as the material removed, so that it is a fairly snug fit inside the opening - there should be about a 1/16-inch gap around its edges. Nail this in place by driving ringed nails into the studs on each side.

The joint between the old and new material is then filled in and smoothed over by using joint cement with the special perforated paper tape that is sold for this purpose in paint stores and lumber yards. This material is applied in a three-step sequence:

1. A wide putty knife is used to smear the compound over the joint so that the seam is filled and an area about 2 inches wide is covered with the compound on each side.

2. The paper tape is then pressed into this with the same knife, smoothing off excess as you go so that the tape is only partially embedded in the wet compound. Try to feather out the edges along each side so no lumps or ridges remain.

3. After the first application has dried, a second application of the compound (without the tape) is applied over the top, feathering it out on each side so the seam is about 5 to 6 inches wide. Again smooth out carefully to leave no ridges or bumps, and let dry.

In some cases a light sanding will be required to finish the seam, but in the event there are still shallow areas or uneven sections, a third application of compound may be advisable before sanding.