Varnish

Varnish is a film finish built from drying oils and resins dissolved together, curing through a slow oxidative chemical reaction with the air rather than simple solvent evaporation -- which is why varnish takes far longer to dry and cure than lacquer or shellac, often a full day or more between coats. That slower chemistry produces a genuinely tougher, more UV- and weather-resistant film, which is why spar varnish (formulated with extra flexibility and UV protection) is the traditional choice for exterior brightwork on boats.

Polyurethane is, technically, a specific type of varnish -- a synthetic resin formulated into the same oil-and-resin curing system -- and has largely displaced traditional varnish for general interior furniture and floor use, while true varnish still holds its ground for exterior and marine work where its flexibility and UV resistance matter more.