Painters and furniture finishers hardly ever apply polyurethane with a roller because of its propensity for forming bubbles. They can form even when you use a high-quality paintbrush, however, and if you don't catch them while they're fresh, they become part of the finish. It isn't difficult to avoid making them in the first place -- it just takes a gentle touch when stirring the polyurethane and dipping your brush in it.
Bubbles in Fresh Finish
Two axioms for furniture finishers are that you shouldn't shake the can before applying polyurethane and you shouldn't wipe the brush along the side of the can. Both activities introduce bubbles into the mixture in the can, and it's likely that you'll transfer some of these to the surface you're finishing. Whether or not you have bubbles in the can, though, bubbles in the finish are almost inevitable -- the friction of the brush against the surface creates them. As soon as you notice them:
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- Dab the bubbles lightly with the tip of the brush.
- Run the brush very lightly along the surface to flatten the bubbles into the finish.
Bubbles in Hardened Finish
If you missed the bubbles while the varnish was fresh, you may find the finish mottled with tiny craters or hardened bubbles. This happens to floor finishers who drag the finish applicator aggressively and then must stay off the floor until the finish dries. The only remedy is to sand out the bubbles and apply another coat of finish -- more carefully this time.
Sanding Out Bubbles
Scuff-sand the bubbles with 120-grit or finer sandpaper, depending on what you're finishing. You would use coarser paper to scuff a floor than you would a tabletop. If the bubbling is extensive, sand with a palm sander or, if you're finishing a floor, a floor buffer and a sanding screen.
Wipe away the sanding dust with a damp cloth.
Apply a fresh coat of finish, moving the applicator slowly to avoid turbulence and more bubbles.