Polyurethane is a durable clear sealant available to the woodworker. It is sold at home centers and comes in wipe-on on normal varieties. It can be recoated in 4-6 hours. It takes at least 3 coats of polyurethane to produce a reasonable finish. Often several more coats are applied to achieve a high class finish.
True Polyurethane is oil based. The water based variety is a relatively new product. This article is derived from experience with oil based polyurethane.
The normal variety of polyurethane is thicker than the wipe-kind. It builds faster, but is more susceptible to drips. It takes more coats of wipe-on poly to produce the same thickness fewer coats of polyurethane achieve.
1) Poly is applied with a foam brush or a piece of cloth for the wipe on variety. It is imperative that you do not to apply too much poly in a given area. This will cause drips to appear in the finish, which are hard to remove.
Resist the urge to apply a second coat before the first one dries. This will cause drips.
2) After the first dries it must be sanded with 220 or 300 grit sandpaper. This will produce a lot of fine white dust and make the surface appear dull. The fine scratches will disappear when the next coat of poly is applied. When you do this the first time it looks like the sand paper is ruining the piece. That is not so.
Take care not to sand through the poly. This will remove any stains that were applied to the piece before the first coat of poly was applied. This is particularly easy to do when using wipe-on poly.
3) Remove all the fine dust from the piece using a tack cloth. Apply a second coat.
4) After the second coat had time to dry, inspect the piece. If you can feel bumps that should not be there, sand the piece again and apply another coat. Repeat these steps until the piece feels smooth.
5) Apply as many coats after the initial coats as you like. The last few coats even out better if the poly is thinned with mineral spirits. Even wipe-on poly can be thinned this way.
6) Finally buff the surface to the shine you want.
Both kinds of poly can be thinned with mineral spirits. This creates a version that levels out faster, but is more susceptible to drips. Normal poly that has been thinned this way can be used as wipe on poly. Thinned poly can be used with spray guns.
Minwax is probably the most widely available brand in North America.