Apply the finish in a clean, well-ventilated area. Polyurethane takes hours, not minutes, to dry; that's a lot of time for dust to settle or bugs to land on the surface, marring the final product. Both water-based and oil-based products give off strong fumes as they dry (although oil-based is decidedly worse), so proper ventilation is a must. Start by finish-sanding to at least 180 grit, then remove all sanding dust with a clean rag or shop vacuum. Spray on one or two light coats, let the finish dry thoroughly, and then rub it out (buff smooth) with 320- or finer grit sandpaper. If you’ve stained the wood, spray two seal coats before rubbing out to avoid sanding through the stain.
Like polyurethane, Varnish is a curing finish. Some varnishes may even contain polyurethane. They are very similar and some people even use the phrases interchangeably. Varnish is typically a glossy finish, but flattening agents can be added. Be sure to stir well when using these flattening agents.
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