When a picture frame starts to work loose at the corners, or when its finish gets scratched or starts to crack and chip, most people throw it away and buy a new one. While this may be the most practical solution when the frame is inexpensive, replacing one that is ornate, possibly carved or gilded, can prove costly and difficult.

Yet in many cases repairing and renovating the old frame is well within the capabilities of a do-it-yourselfer who is willing to take the time and trouble to tackle the job. The work can be done with easily available materials sold in most paint and art supply stores, and in most cases no special skills are required. For most beginners, however, it is a good idea to practice on an old frame before trying to work on a more valued piece.

You will find, as you gain experience, great satisfaction in prolonging the life of hard-to-replace and possibly expensive frames, and you will be able to rejuvenate or reclaim old frames that may have been stored away for years in attics or basements -- or those you can find at bargain prices in flea markets and garage sales.

Physical repairs, if needed, are best done before refinishing. This means regluing loose joints, repairing cracks or splits, and patching or replacing missing or chipped pieces on carved moldings.

If a corner joint is loose, pry the joint open slightly, then use a pocketknife or similar tool to scrape out as much of the old glue as is practical. In most cases you will have to do at least two joints or corners, because prying one joint open usually causes an adjacent one to open. Apply white glue with a palette knife or similar thin spatula, then clamp the joint firmly back together and drive in a very small brad to hold it.

Although this can be done in an ordinary vise, the best way to clamp the pieces into perfect alignment (right angles) while the glue sets is to use a special picture frame corner clamp. This is a vise with two sets of jaws that clamp each piece separately, holding the ends together at a precise 90-degree angle. Various models are sold in most well-stocked hardware stores and in shops that sell framing supplies.

Small cracks or splits in the frame can be repaired by working glue into the crack with a palette knife or the tip of a toothpick and allowing it to seep down into the crevice. Then bind or clamp to apply pressure while the glue sets. Don't use nails or brads -- this may cause more splitting.

Heavily carved or ornate molded frames may be chipped or have pieces missing -- usually along the edges or in one of the corners. Reasonably good repairs can be made by rebuilding the damaged sections with artist's gesso -- a pastelike modeling compound resembling plaster that dries hard and provides an excellent base for gilding and finishing when smoothed over. Gesso is sold in art supply stores.

To make such repairs, clean the surface first, then press a blob of gesso paste over the missing area. Use a small knife blade, palette knife, toothpick or similar tool to carve and shape the material so it blends artfully with the rest of the pattern.

If the repair is sizable it is best to build the new section in layers, allowing each one to dry before going to the next.

When you have the built-up section shaped as closely as possible to the missing parts, allow it to dry hard, then sand carefully with small pieces of abrasive paper wrapped around your finger. Use small files to reach places your finger cannot get to. A nail file or emery board will be useful for final shaping.

Especially handy for jobs of this kind is a motor-driven hand grinder that can be fitted with a variety of small burrs, grinding bits and other shaping tools. When the shaping is done, apply a coat of orange shellac to seal the surface and prepare the frame for the final finishing.

Since frames with ornate carvings are often finished in gold, the next step will be to regild. Some frames will have been finished with real gold leaf, others with powdered gilding or gilt paint.

Gold leaf is now costly and hard to find, and it takes a fair amount of skill and experience to work with. Excellent results can be obtained by using a good grade of gold-leaf paint or paste-type gold finishes that can be rubbed on like shoe polish. These are sold in art supply stores.

Whether the gold if being applied over wood or gesso, the first step is to clean the surface carefully, then brush on a coat of orange shellac. When this dries, brush on one coat of gold-leaf paint, using a soft artist's brush, then allow this to dry hard. Now rub on the wax-type gold paste, using a piece of cloth wrapped around one finger.

The paste-type gold wax comes in a variety of gilt shades, so select one that most closely matches the one on the frame. Colors can be blended to get a more exact match. Apply the paste sparingly, spreading it evenly over the surface, then take a clean cloth and buff lightly to bring up the luster.

If the frame is stained or colored, use small cans of pigmented wood stain, or thinned-down paint of the right color, to tough up scratches and other blemishes. Experiment on scrap material or on the back of the frame to check the color. Apply the stain or paint sparingly with a small artist's brush. Use a small piece of cloth to remove the excess.