Polyurethane is better known as varnish. Varnish is used to protect and add a shiny finish to wooden surfaces. Over time, the finish of the varnish can start to peel, become white, and even feel tacky. If this has happened to your varnished wood finish, you can repair it. Whether your varnished wooden surface is a floor, door, or even a piece of furniture, it can easily be fixed to look like new again.
Take the wooden item outside to an open area. Sanding creates dust that can be harmful if breathed.
Put on the face mask. Start sanding down the varnish finish with the electric sander. You want to sand the wooden surface down to bare wood. Make sure you sand throughout the entire varnish finish to leave the surface smooth for the new varnish.
Brush off all the loose debris and dust on the surface, using the dry paintbrush. Pour a small amount of white spirit on the cloth and wipe the wooden surface to remove all the remaining dust.
Read the directions on the varnish can for exact thinning instructions. Thin the varnish according to the manufacturer's directions with the white spirit. Mix thoroughly with a paint mixing stick.
Paint the varnish onto the wooden surface, either with or against the grain of the wood. Apply a light coat of varnish, with each successive pass of the brush overlapping only slightly. Do not brush over the varnished areas again while the varnish is still wet. Let the surface dry for at least 24 hours.
Sand the wooden surface lightly with the 320-grit sandpaper. The goal is to roughen the surface of the varnish, not remove the varnish from the wooden surface. Clean up the surface with a dry brush and then the cloth moistened with white spirit. Apply two to three more coats of varnish, following the same process between each coat.