Polyurethane is a clear protective coating applied to many products, including wooden floors. It can also be used as a substance in its own right for manufacturing other items.
The polyurethane coating for timber floors comes in two general types. There are water based coatings and solvent based coatings.
Water based polyurethane is highly transparent. Timber floors will need several layers of water based polyurethane (usually 4 or more) but the results are almost invisible and the protective properties of the coating are very dependable. The fact that this water based coating dries in about 2 hours means that the floor can be given several coatings in one day, and be ready for use the day after.
This solvent based coating is not as transparent as the water based variety, and it will age with a pleasant amber brown hue. But many home owners quite like this look on wood.
Solvent based polyurethane is quite thick, so a floor will only need two coats. But the coats take 24 hours to dry so the process is slow, and the rooms cannot be occupied because of the fumes. Yet the finish is quite pleasing and offers solid protection against floor damage.
Many installers like to re-sand the floor between coats of polyurethane to obtain the smoothest finish possible.