It's natural for some parts of a floor to wear out before others -- high-traffic areas are usually the first to go, and other areas may be victims of scraping furniture legs or pet claws, spills or extra-high humidity. Refinishing a floor is a big deal, and if the floor has only one or two worn areas, while the rest of the finish is in good condition, it's more economical to make spot repairs. Done correctly, a spot repair is all but unnoticeable, provided you've correctly matched the finish and -- if necessary -- the stain.
Test the Finish
A number of tests can help you determine if the finish has actually worn through, whether or not the floor is waxed and what type of finish you're dealing with.
Is It Oil?
Rub your finger along the surface of the floor. If it feels rough, it's probably finished with a penetrating oil, which may or may not be coated with wax. If it's smooth, it may still be well-waxed oil, but probably not. It's much more likely to be a film finish, such as shellac or polyurethane.
Is It Worn Through to the Wood?
Soak a rag with water and let a few drops fall on the worn spot you're concerned about. If the water beads up, the finish may be thin, but it's still intact. If the water soaks into the wood, the finish is worn through. You can skip this test if the stain color is worn -- that's a sure sign that the finish has worn through.
Is the Floor Waxed?
Moisten a rag with mineral spirits and rub the finish vigorously, then look at the rag. Significant darkening means that the floor has a wax finish that you must remove before treating the spot.
Is the Finish Lacquer, Shellac or Varnish?
Evaporative finishes, such as lacquer and shellac, aren't as common as reactive finishes, such as alkyd varnish or polyurethane, but you handle them differently. Put a few drops of lacquer thinner on an inconspicuous part of the floor, then feel the finish -- if it's sticky, it's lacquer or shellac. Repeat the test with denatured alcohol, and if it's still sticky, it's shellac and not lacquer. If toluol or xylene -- both available at paint stores -- soften the finish, it's a water-based lacquer. If no solvent softens the finish -- which is most likely -- it's an alkyd or polyurethane varnish.
If you've determined that the floor is waxed, you may want to consider removing the wax from the entire floor before beginning the repair. Periodic removal is recommended and, well, there's no time like the present. Otherwise, you definitely have to remove the wax from the area you're refurbishing, or the new finish won't adhere.
Remove wax with a rag moistened with odorless mineral spirits. Rub the section of the floor from which you're removing the wax once, then turn the rag around, moisten it and rub again. Continue until the rag shows no discoloration.
Scuff the Finish
Scuff the worn spot with sandpaper. Use 150-grit paper if the finish isn't completely worn through or if the floor is finished with oil. Use 120-grit paper to sand the finish around a spot from which it has completely worn. The coarser paper will cut back the finish more efficiently. Sand with the grain and create a scuffed area significantly larger than the spot you're repairing.
Restore color to the worn area, if necessary, by wiping on stain. If you have some leftover stain from the last floor refinishing job, use that or a closely matching commercial stain. You can also mix your own stain by adding pigments to lacquer thinner. Let the stain dry for several hours.
Apply the appropriate finish to the spot, using a rag or paintbrush and stroking with the grain of the wood. A paintbrush gives more control when using varnish, shellac or lacquer, but use a rag if applying oil. Let the finish dry, then scuff it with 150-grit sandpaper and apply another coat. Repeat if necessary.