A. If there is a gap in the finish, you should be able to feel the textural difference with your fingers. If you can and the vanity is in a powder room, you can probably patch the area with dewaxed shellac, which Zinsser sells as SealCoat Universal Sanding Sealer. Let that dry, then see if the color matches. If not, rub off the shellac with a cloth moistened with denatured alcohol, then dab on a bit of an oil finish. Let that dry completely, for at least a day, then reapply the shellac. If the color matches, add more coats of shellac until the patch is level with the surrounding area. Shellac dries quickly, so you can put on multiple coats in a day. Then, if you want, you can scuff-sand the entire countertop and brush on a coat of polyurethane to give everything a uniform shine. Because the countertop was finished a long time ago, you shouldn’t have to worry about whether the poly will stick, regardless of what the initial coats were. (Why specify dewaxed shellac for this approach? If you used standard shellac, which isn’t dewaxed, a topcoat of polyurethane might peel.)
However, if the vanity is in a bathroom with a shower, patching with shellac isn’t a good idea because the fluctuating humidity during and after showers will make the wood swell and shrink a lot, causing the shellac patch to peel off. Instead rub a small area with a cloth moistened with acetone or acetone-based nail polish remover. If the finish softens, touch up with lacquer. If the finish stays intact, use polyurethane, or for a better-looking job, strip off the existing finish and start over. You’ll need to take out the sink to do a good job, though.
If the finish feels smooth, what you’re seeing is probably white spots, which form when moisture gets into a finish. White spots can sometimes be drawn out of a finish by applying something oily, such as butter, mayonnaise or furniture polish with lemon oil. Or you can use gentle heat from a hair dryer set on low. Move the hair dryer constantly so you don’t get any spot too hot.
If the spots persist, the moisture probably caused minuscule cracks in the finish. To burnish and fill them, try rubbing with a commercial ring remover, which contains a fine abrasive. Or rub with 0000 steel wool and dark brown wax.
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Read Jeanne Huber’s roundup of home-improvement tasks you should tackle in August, such as upgrading locks.