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Polyurethane is so durable and water-resistant, it has largely replaced shellac and varnish as a wood finish. Originally, it had to be brushed on, but different formulations mean it can now be applied as a spray or by wiping it on with a rag. Here’s how to gauge which approach is best for your project.

Brush-on Great for broad, flat, horizontal surfaces—floors, tabletops, chair seats. Forms a thicker coat, so the finish builds up fastest. Polyurethane coatings brushed on vertical or contoured surfaces are more likely to drip and run. Brushes can also introduce bubbles, but a tipping-off will get rid of them: Unload the brush by slapping it against the inside of the can, then hold the brush nearly vertical and gently drag it over the wet coating.

Wipe-on This less-viscous poly is best for round, vertical, or contoured surfaces—crown molding, wainscoting, stair balusters—where brushing might create drips. Each wipe-on coat is thinner than one brushed on, so four coats are needed for good protection.

Spray Handy when coating hard-to-reach surfaces, such as shutter louvers and chair spindles, or encapsulating flaking paint on flea-market finds. Or use it to touch up a scratched or damaged finish. Sprays require careful application and some practice to avoid drips.