Spray on the Final Coat
Here's a trick for getting a glass-smooth finish on your next woodworking project. Start by brushing on a coat of gloss polyurethane. Let it dry overnight. Then lightly sand with 320-grit sandpaper to remove imperfections. Use a tack cloth or vacuum cleaner and soft brush attachment to remove the dust. Repeat this process for the second coat. Finish up by spraying on the final coat. You can buy aerosol cans of polyurethane in satin, semigloss and gloss finishes. Any of these can go over the gloss coats.
Brushing on the first two coats allows you to build up a thicker layer of finish with less cost and effort than spraying from cans. And using an aerosol can to apply the final coat produces a professional-looking finish, free of brush marks.
Don't Sand Through the Stain
When sanding between coats, it's easy to rub right through the clear coat, removing the stain below. So sand super lightly after the first coat, just enough to cut down any dust whiskers on the surface. If there are bigger problems—such as runs—deal with them after the second coat when you can sand a bit harder. To repair rubbed-through spots, just apply new stain. Immediately wipe away any stain that gets on the surrounding polyurethane.