All About Polyurethane

It's a kind of super-tough varnish formulated so that its microscopic chains of resin molecules will bond tightly with one another as it dries. The result is a finish that's much more resistant to water, solvents, abrasion, and impacts than traditional varnishes.

Some polys have oils that give wood a warm, amber tone. If you want wood to keep its light color, use a water-based poly.