While a crack in polyurethane wood flooring is unsightly, it's better than a crack in the wood flooring beneath it. A crack in wood flooring is a more serious, fundamental problem, whereas a crack in the finish is superficial. However, it is still important to repair the crack so the wood remains completely protected and to prevent the crack from getting bigger.
Things You'll Need
2 2-inch paintbrushes
2 wads #0000 steel wool
Pour 2 or 3 inches of denatured alcohol into a wide-mouthed jar. Dip a 2-inch wide bristle brush into the jar.
Wipe the brush and denatured alcohol onto the cracked areas of the polyurethane with light strokes. Watch the finish carefully. Apply two or three more coats of the denatured alcohol until the finish softens and begins to flow smoothly. You'll know when this is happening when the cracks begin to disappear.
Rub the area of the crack with #0000 steel wool, following the direction of the grain. Dip a 2-inch brush in polyurethane and apply a very light coat over the formerly cracked area. Allow it to dry overnight.
Buff the area with #0000 steel wool dipped in paste wax. Buff the surface vigorously with a lint-free cloth.