Polyurethane is a very tough, durable varnish that coats a piece of wood with a plastic-like coating. Polyurethane is impenetrable by many substances, even those that are acidic, making it ideal for use on cabinetry. When cleaning a cabinet coated in polyurethane, remember that you are cleaning the finish, not the wood, so using cleaning products suitable for the coating is very important.
Test the Finish
Before cleaning a polyurethane-coated cabinet, test the finish to ensure it is polyurethane. Rub a few drops of boiled linseed oil into the wood. If it beads up, it has a hard finish, not an oil-based one. Rub a little acetone (nail polish remover) over an inconspicuous spot in a small, circular motion. Lacquer will dissolve and varnishes and shellacs will turn sticky after a few minutes. Polyurethane will repel the acetone, and it will bead up like water.
Keep your cabinets dusted well using a dry microfiber cloth. Brush very dusty areas or built-up dust off the cabinets with a stiff, nylon brush. Brush in the grooves of the cabinet gently. Use short, gentle strokes to remove the dust. A small toothbrush can be used to gently remove dust in crevices. A vacuum with a brush attachment can also be used to remove dust from cabinets.
Grease and Grime
When cleaning anything covered in polyurethane, remember that you are cleaning the finish, not the wood. Waxes and polishes can build up and become a problem later on. Mineral spirits on a clean rag will remove most smudges and dirt. A mixture of water and vinegar in a spray bottle can be used to remove dirt and greasy marks on your cabinets. Greasy marks can get all over a cabinet whether you're cooking or there's just a lot of use. Vinegar breaks up the grease without damaging, causing any abrasion to the polyurethane. Because vinegar works like a glass cleaner, it also won't cloud the finish like other cleaners and can break through any buildup. Rubbing alcohol applied in small amounts to a clean, dry cloth can also be used to rub away grease and grime, especially those found around knobs.
One of the best ways to get rid of stains on a polyurethane cabinet is to use a microfiber sponge and rub the stain away. Permanent marker, crayon and other stains are easily removed this way. If the stain has discolored the polyurethane, you can sand the stained area off with a fine grit sandpaper. Wipe away the dust left behind with a tack cloth and reapply a coat of polyurethane. Let it dry, and sand and reapply if it needs it. If the stain splattered or covered a large area, take the door off if the stain is on the door and refinish it completely to get a uniform look. If the staining is on the cabinet itself, refinish that side.