Sanding-down dried wood fillers such as Zar, Elmer's, etc. can be time consuming; also, if you're working on antiques, you can remove patina and leave light spots. Fortunately, I have found that althought the filler dries hard, it never becomes water resistant. A folded and moistened paper towel or rag wiped across the dried filler will remove it right down to a level surface much quicker than you can sand it. Sometimes I do this before staining; however, I sometimes wait until after one coat of finish before flling and leveling, so I can color-match the filled spots using artists oil pigments.