Polyurethane not only adds gloss and shine to wood surfaces, it also provides protection against wear and the elements. Although polyurethane tends to eventually begin turning yellow, it is still often preferred over varnish because of its durability. It does take some special care to get a good finish, such as properly preparing the surface and taking your time during application, but the effort is worth it, especially if you're looking for a strong, high-gloss finish.
Although it goes by many different names, the basic types of polyurethane are water-based and oil based. Each has its advantages, but most experts advise using water-based polyurethane on items that will be inside and oil-based on those that will be outside.
Water-based polyurethane is easier to clean up, has less odor, is more environmentally friendly and doesn't turn yellow as easily as oil-based products.
Oil-based polyurethane stands up to harsher conditions, lasts longer and wears better, but also tends to yellow more over time, can have strong odor and is highly flammable.
Any surface where polyurethane is applied should be clean and free of dust, dirt and lint. The best way to prepare wood is to wipe it down thoroughly with denatured alcohol prior to application. Be sure to wait until it is completely dry before applying the polyurethane.
Surfaces will also need to sealed. Most sealers are simply a diluted version of the polyurethane you are using. Check on the can for directions for diluting the polyurethane to create a sealer.
Avoiding air bubbles and visible brush marks are the keys to a beautiful polyurethane finish. Mix the polyurethane carefully, taking care not to move the stick up and down to add air bubbles, and don't shake the can. After dipping the brush, avoid the temptation to wipe it on the edge of the can, which can also add air bubbles.
Use a high-quality natural bristle brush. An investment in a good brush pays off with a nice smooth finish. Use long, even strokes to avoid brush marks. Apply thin, even coats.
Sanding between coats and cleaning away any resulting dust is vital for a smooth, clear finish. By sanding, you create a roughened surface for the next coat of polyurethane.