Brush marks often show up on pieces of furniture treated with polyurethane finish, usually despite the best efforts of the craftsman. However, there is a way to remedy these flaws without undoing a day's work. Sanding removes the lines and ridges created by the brush marks without ruining the finish on the whole piece. A light coating of finish after sanding disguises the area.
Sand down the brush marks lightly with the fine-grit sandpaper. Make sure to apply even pressure to the area.
Brush the sanded area off with a soft cloth to remove all traces of dirt and grit from the sanding. These particles can get caught under the finish and ruin it.
Apply the polyurethane finish lightly to the brush, making sure to wipe off any extra. This will make sure that the area doesn't get too much finish.
Apply the finish gently to the sanded area, with as few brush strokes as possible. It may take several light coats to create the desired effect, but that's better than applying too much. If the coating looks too thick, use the rag to lightly swipe at the area until it is even. Allow the polyurethane finish to dry.
Inspect the dried area for brush marks. If some remain, re-sand the furniture.