Use sand paper to remove the original finish from the wood. Depending on the wood, you will need different kinds of sandpaper. Begin with a 100 grit (medium) sandpaper and then use a 150 grit (fine) to prepare the wood for finish. The wood surface should be uniform and free of finish after sanding.
Use a vacuum to remove any surface dust or dirt from the wood. Open the polyurethane bucket and use a stirring stick to mix the polyurethane. Stir from the bottom of the can to prevent the thicker material from settling at the bottom.
Dip your paintbrush into the can so that 1/3 of the brush is covered with the polyurethane. Apply a thin coat of the polyurethane to the wood. Brush the polyurethane in smooth strokes so it does not clump on the wood surface. Allow the substance to dry for about 4-6 hours between coats.
Use a fine-grit sandpaper to even out the finish on the wood after the polyurethane has dried. Use a 150-220 grit sandpaper for this. Use your vacuum cleaner to remove any dust that has accumulated after sanding.
Apply a second or third coat if needed. Allow the wood to sit for about 24 hours before you use it. Use paint thinner to clean up any spills or spots that occur during use and to clean your brush. Put the lid back onto the can and snap it into place.