How to Refinish Varnished Mahogany Boats With Polyurethane

Apply chemical stripper to the boat's finish with towels or rags according to the package directions. Place the chemical stripper on any areas that have the old varnish. Most types of varnish cannot be covered over with new polyurethane, so if you are unsure about the original varnish on your boat, then strip it. If your original varnish is one that can be covered with new polyurethane, you can skip this step.

Remove the old varnish from the wood using the chemical stripper. Allow the wood to dry completely.

Sand the wood with 220-grit sandpaper. Do this if you removed old varnish or if you are covering it up.

Clean the area with a vacuum cleaner or tack cloth. Make sure you remove all sanding dust.

Apply the polyurethane varnish to the boat's surface using disposable foam brushes. To apply the varnish, use long, even brush strokes and ensure you always have a good amount of varnish on the brush. Begin with your brush in a dry area and gently brush towards a wet area, where the varnish has already been applied. When you reach the wet area, lift the brush up with one even stroke. Dragging a corner of the brush in the varnish can leave an uneven finish.

Allow the varnish to dry. Once it has set completely, sand the area lightly with 300 grit or higher sandpaper. You can also clean the area with a lacquer thinner. If you sand the boat, make sure you remove all the dust. If you use the lacquer thinner, allow it to dry completely.

Add a second coat of polyurethane, allow it to dry and then clean it. You should plan on at least three coats of varnish for interior wood and five coats for wood that will be in the water, according to Boat Wood Finishing.