Urethane is a great sealer and protector. It comes in many forms from polyurethane to urethane-enhanced paint. You can use it on small items, on floors, furniture and tables. The quality of your finished result depends on how the urethane is applied.
Sand the surface you wish to finish with urethane. Use a fine-grit sanding block or sand paper. For large items like tables, use an electric hand-held sander. Sanding removes the old finish and helps remove any nicks, dents or scratches. The finish of the urethane will be much smoother if you sand. Use a soft cloth to remove all of the dust left from sanding. If there is any remaining dust the urethane finish will not be smooth.
Do not shake or stir the urethane. When you shake or stir urethane, it creates tiny air bubbles. The bubbles will then be transferred to the surface you are refinishing. The only way to remove the bubbles is to allow the urethane to dry and then to sand off the bubbles.
Apply urethane with a clean roller or soft paint brush. You will get the best results if you use a brand new brush or roller. Paint brushes are great for smaller surfaces and rollers work well for floors. Use a soft-bristle or foam brush for best results. If you use a roller and tray, pour the urethane slowly to avoid air bubbles. Allow to dry 24 hours.
Sand the surface with a fine-grit sand paper. Sand it lightly, with a piece of sand paper, not a sanding block. Do not apply pressure. Dust off.
Apply a second coat of urethane. Sanding the wood helps the first coat of urethane adhere. Sanding the first coat, when dry, helps the second coat adhere to the first. The layers of urethane will not peel apart. They will not chip, or have ripples or bubbles, because the first coat was sanded. The second coat is the final coat. It will be smooth, glossy and even. Do not sand the second coat. Allow the second coat to dry for 24 hours before using the your refinished item.