During application of a finish, whether by rag, brush or spray, there are sometimes problems so severe that you would like to remove what you have just applied and start over. You can do this fairly easily with oil and varnish finishes, with somewhat more difficulty using water-based finishes, and not at all with shellac and lacquer finishes without removing all previous coats down to the wood.
With oil finishes, you may have as long as an hour to remove the finish without damaging the coats below. Use mineral spirits (paint thinner).
With varnish (including polyurethane varnish), you may have 15-to-30 minutes before the finish sets up too much. Use mineral spirits.
With water-based finish, you have to be very quick; you have only a few minutes. But you won’t damage dried coats below as long as you use water.
With each of these cases, wet the rag well—that is, soaking wet—and turn the cloth often.
With shellac and lacquer, you can’t remove the last coat applied without damaging and removing all the coats below. The solution, if you have a problem you want to remove, is to let the finish dry, then sand out the problem.
You could also strip all the finish and start over, of course, just as with any finish.
