How to Finish New Wood on Windows & Doors: Polyurethane

Sand the window frame or door using 100-grit sandpaper. Follow up with 150-grit sandpaper, and finally with an extra fine-grit sandpaper such as 220-grit. Wipe away the dust particles using a tack cloth.

Create a sealant using one part mineral spirits and two parts polyurethane. Pour the mixture into a glass jar and stir it with a wooden stick.

Dip a paint brush into the mixture to a depth of one inch and apply the seal in the direction of the wood grain. Use long, even strokes and overlap each stroke with the previous one. Overlapping strokes prevents obvious lines and ensures that you won't miss a spot. Inspect the wood for drips once you are complete; blend drips into the wood with your brush.

Allow the sealant to dry until it is no longer tacky to the touch. Apply one coat of polyurethane straight from the can -- do not wipe excess finish on the edge of the can or air bubbles may be created on the brush. Use long, even strokes to apply the polyurethane and work in the direction of the wood grain, overlapping the strokes. Allow the finish to dry for 24 hours and then add a second coat. Let the piece dry for another 24 hours.

Inspect the wood one final time for any drips that you may have missed. Scrape existing drips from the surface using a razor blade.