Move the desk outdoors, if possible, to minimize exposure to chemical fumes. Place the furniture on old newspapers or a disposable plastic drop cloth to catch polyurethane spills and splatters.
Sand all surfaces of the desk until smooth with progressively finer-grit sandpaper.
Brush off wood dust from the desk and wipe down all surfaces with a clean cloth dipped in mineral spirits.
Stain the desk, if desired, using a stain color of your choice. Apply with long, even brush strokes flowing with the wood grain. Let the stain dry 24 hours before applying sealant.
Mix 2 parts polyurethane to 1 part mineral spirits in a small bucket to prepare a homemade sealant. Diluting the polyurethane with mineral spirits helps it absorb better into the wood. This seals the wood in preparation for the final polyurethane coats.
Apply one coat of the sealant in long, even brush strokes, flowing with the wood grain, to all desk surfaces. Long, smooth strokes minimize bubbles in the finish. Let the sealant dry.
Apply polyurethane to the desk with long, even brush strokes flowing in the direction of the wood grain. Leave the desk to dry for 24 hours after applying the first coat of polyurethane. Apply a second coat of polyurethane after the first has dried completely.