Decide whether you want a water- or oil-based polyurethane. Oil-based products are generally rated as stronger and longer lasting than water-based products. They are especially useful for furniture that will be used outdoors or exposed to moisture and humidity. Water-based polyurethanes rarely yellow or discolor the finish of furniture. They often contain a less harsh scent than oil-based products.
Prepare the surface of the furniture you want to seal. A polyurethane coat is only as good as the prep work that precedes it. Sand down the area with fine-grit sandpaper to help the varnish adhere properly. This will also remove imperfections that might be more noticeable after the varnish has dried. Remove any dust or dirt with a warm, damp washcloth.
Apply the polyurethane with the grain of your furniture, using a sponge brush. Dip the brush halfway into the can and scrape it against the side of the can to push off excess liquid. Apply an even layer over the furniture's surface.
Allow your first coat to dry completely. If you see imperfections in the wood, you might sand it down again and start over. Again, good and even sanding is the only way to ensure the best results with your polyurethane finish. Allow each coat to dry before adding another coat.