How to Apply Polyurethane: When working with any finish...
Choose the right brush.
Use natural bristle for oil-based finishes and synthetic filament (polyester, nylon, or a blend of the two) for water-based and water-based oil-modified finishes. Don't use rollers or foam brushes—they create bubbles.
Dampen the brush with a solvent.
Your brush will be easier to clean, and will go longer between cleanings, if its bristles are dipped first in mineral spirits, if you're using an oil-based poly, or in water, if the finish is water-based. Before using the brush, rapidly roll the handle between your hands inside a cardboard box to eliminate excess.
Pour some finish into a clean container.
Working from a separate container prevents the dust and other particles picked up by the brush from contaminating the finish in the can.
Tap, don't wipe.
After dipping about one-third of the brush into the finish, gently slap it against the inside of the container. Scraping the bristles over the edge of a container leaves the brush too dry to apply a coat of the right thickness.
Always brush or wipe with the grain.
It's the best way to work the finish into the wood pores and ensure an even appearance.
When working with water-based finishes...
Dampen the wood first.
Wipe the wood with a damp cloth to raise the fibers; after the surface dries, knock them back with 220-grit sandpaper.
Apply and move on.
Too much brushing can leave these fast-drying polys rough and ragged.
Don't smooth with steel wool.
It leaves behind tiny bits of steel that you can't see—until they rust. Use sandpaper instead.