How to Polyurethane a Door

Apply paint or stain on the door in the desired color. Allow the paint or stain to dry for the length of time recommended by the manufacturer, or at least 24 hours, before proceeding.

Sand the bare wood surface if the door is not painted or not stained, to make it ready to receive the polyurethane. Begin with 180-grit sandpaper. Sand the entire door, making sanding strokes along the grain of the wood, not perpendicular to it. Re-sand the door twice more, first with 200-grit sand paper, and finally with 220-grit sandpaper.

Vacuum the sanding dust, using a brush attachment for your vacuum cleaner. After the dust in the air and area settles, remove any remaining dust specs with a tack cloth or a well wrung-out, soft cloth.

Prepare the door supports by driving 3-inch wood screws into 1/2-inch-thick scraps of lumber. The scraps should be approximately 3 to 4 inches square. Drive the screws directly into the center of the scraps, all the way through the wood until the head of the screw prevents it from turning with a screwdriver.

Place the door on the painting surface so the backside of the door is on top. Brush on polyurethane, following the grain of the wood as much as possible. Do not keep going over areas where you've already applied polyurethane because it may cause brush strokes that will not sand out.

Lay the supports on the surface of your work bench or area on the floor where you will put the door while applying the polyurethane to the other side of the door. Orient the supports so the tops of the screws are facing down. The door will rest on the points of the screws, leaving marks in the polyurethane that you can easily sand out. Turn the door over and carefully position it on the supporting screws.

Apply polyurethane to the other side of the door, along with the side, top and bottom edges. Allow the polyurethane to dry the length of time recommended by the manufacturer.

Sand the first coat very lightly, using 220-grit sandpaper or extra-fine steel wool, after the polyurethane is sufficiently hardened. Remove sanding dust first with a vacuum cleaner and then with a tack cloth.

Apply two additional coats of polyurethane, sanding after each coat in the same manner as before.