Things You'll Need
Medium and fine sandpaper
A wooden cabinet in a bathroom is subjected to heavy moisture in the air that can damage the wood surface over time. Most cabinets come with a protective finish of paint or varnish, which may not be enough protection for a bathroom environment. Coating the piece in polyurethane will add protection without obscuring the color of the surface. Although oil-based polyurethane has a slower drying time than water-based products, it provides better protection.
Disassemble the cabinet, taking off all hinges and hardware with your screwdriver. Lay out the pieces on old newspapers.
Set up your sander with medium-grit sandpaper. Run the sander over the surface cabinet parts, dulling each section to take off any gloss or grime.
Repeat the process with fine sandpaper to get the piece smooth and clean. Wipe up the dust with a tack cloth.
Brush a light, smooth coat of oil-based polyurethane on the cabinet sections, using a refinishing paintbrush. Keep the polyurethane thin and smooth, then let it set for 24 hours.
Sand the dried polyurethane lightly by hand with the fine sandpaper, just enough to make the surface dull and flat. Wipe up the dust with the tack cloth.
Apply a second layer of polyurethane. Let it set for 24 hours, hand-sand it, then brush on a third layer. Let the third layer set for 48 hours. Put the cabinet back together.