Ordinary use, abuse and the ravages of time can damage a furniture finish, but fortunately most of this damage is easily repaired.
All that's required is some time and a minimum of materials and expense. In most cases, it's better to preserve and repair a finish than it is to replace it (strip it off and refinish it). It's amazing what a difference a thorough cleaning and repair job can make. Also, repairing instead of replacing a damaged finish preserves the character of a piece of furniture.
Most of the tools to make these repairs are available at your local paint store or home center.
The first step is to identify the finish used on the furniture being repaired. Ideally, you'll overcoat the repaired area with the same finish. Shellac, lacquer and varnish are common finishes. To identify them, apply a solvent to an inconspicuous area.
Start by applying denatured alcohol. Alcohol will readily dissolve shellac, and it will slowly soften lacquer. Lacquer thinner will readily dissolve lacquer, and will soften shellac. It will cause varnish to swell, crinkle and lift.
Next, clean the surface using a soft cloth dampened with mineral spirits (also known as paint thinner) or commercial furniture cleaner. This removes built-up dirt and wax and gives you a clearer idea of what the finish actually looks like. After cleaning, you may discover the damage is really just a light scuff. If so, you may be able to hide the scuff by applying paste wax or polish.
You can also hide a minor scratch by rubbing over it with a furniture wax pencil or by applying some liquid touch-up solution. Both are available in a variety of wood colors. Touch-up solution stains and overcoats in one step.
Crazing and alligatoring describe a pattern of fine, irregular cracks in the finish usually caused by excessive heat or long exposure to sunlight.
Alligatored shellac or lacquer usually responds nicely to amalgamation (applying a solvent to partially dissolve the finish). Stroke on the solvent with a fine artist's brush until the finish softens and fills the cracks. Let the finish reharden overnight, then buff over the repair with some paste wax. Amalgamation doesn't work on varnish, however. The condition can be lessened slightly by varnishing over the affected area.
Fill a gouge with wood filler, either premixed or a powder which is mixed with water. Most are buff-colored, while others are wood-toned. Colored compounds rarely match the wood, so you need to stain them.
First scrape away loose finish around the gouge. Next, press the compound firmly into the gouge with a putty knife. Shave off the excess, then sand the filler flush with a sanding block and 320- or 400-grit sandpaper.
Stain the patch with a cotton swab and draw grain lines on the patch with a felt-tip pen when the stain is dry. Marking pens are sold in a variety of colors at art supply stores. Seal the stained patch with a coat of diluted shellac (refer to the thinning instructions on the can), then apply the same finish on the patch as was used on the rest of the piece. Blend in the patch by rubbing it with 4-0 steel wool dampened by mineral spirits.
If you don't know what the original finish is, use padding lacquer, available from most mail-order woodworking supply houses. This lacquer is compatible with any finish.