Before applying a polyurethane coating, a wood surface should be cleaned to remove dust and debris. Sometimes, even with pre-prep cleaning, dust still makes its way into your freshly painted and still wet polyurethane. Dust stuck in the polyurethane gives your wood surface a permanent dirty appearance. Dust trapped underneath the clear polyurethane coating will not wipe away. Minor refinishing is the only cure for this problem.
Things You'll Need
220 grit sandpaper
Mop or cloth
Sand away the dust areas with 220 grit sandpaper.
Smooth out the areas around where you just sanded with sandpaper to make the wood surface even and level.
Clean the wood surface with a mop or cloth and a multisurface cleaner to remove any dust or debris from sanding. Allow the area to dry.
Apply a new coat of polyurethane over the area you sanded. Use a clean foam brush to apply the polyurethane coating. If you did extensive sanding, you might have to re-coat the entire wood surface with polyurethane. If the sanding was minimal, you can get away with just re-coating the sanded area.
Allow the polyurethane coating to dry for 24 hours.