After the finish dries for two weeks (it takes that long for polyurethane to cure hard), wet-sand the finish with a rubber- or felt-backed sanding block starting at 320 or 400 grit. Using mineral oil as a lubricant, as shown, slows the sanding process more than sanding with mineral spirits, produces fewer odors, and helps avoid accidental sand-through.Sand until you have a uniform layer of scratches across the entire surface. Gloss finish helps you spot gaps in your sanding easier than using satin finish.Sanding completely through part of the topcoat and into the one beneath can leave irregular rings. To remove them, wipe away all sanding residue and mineral oil using mineral spirits. Then lightly scuff-sand with 220-grit abrasive for a good bond, apply another topcoat, and allow it to harden.An angled light source helps you check the consistency of your sanding as you level the last coat of finish, shown in photo.