Sealing hardwood floors with polyurethane protects them from spills, scratches and heat while bringing out the depth and color of the wood. But bubbles in the polyurethane caused by agitation, quick application and a thick finish can ruin your results. The air bubbles can get trapped inside the finish and won't rise to the surface without the right technique. Even if you don't have a lot of experience with finishing wood floors, it's possible to have smooth, bubble-free results. The whole project can take at least one full day to complete and even longer to cure, so keep kids and pets out of the area during application and until the floor fully cures to prevent any flaws in the finish.
Clean the floor with mineral spirits on a rag to remove any dust or debris before applying polyurethane.
Stir the polyurethane with a paint stirrer to mix it thoroughly.
Pour the polyurethane into a paint tray.
Roll a lambswool or synthetic paint roller in the polyurethane to saturate it with the finish.
Begin at the farthest point from the entrance of the room. Work quickly from one end to the other, applying the polyurethane in a thin, even coat. Work back and forth and maintain a wet edge, but avoid overworking the same spots to prevent excess bubbling. Continue this until all of the floor is covered completely. Allow the first coat to dry for 4 to 8 hours, depending on the specific formula of polyurethane you're using.
Sand the floor lightly but thoroughly with 220-grit sandpaper to smooth out any bubbles or imperfections in the finish. For very small imperfections, use 320- or 400-grit sandpaper. Follow the direction of the wood grain when sanding.
Dampen a rag with mineral spirits and wipe the floor to clean up all traces of sanding dust.
Mix 1 part mineral spirits per 10 parts polyurethane in a can and stir it with a paint stirrer. This thins out the polyurethane and gives the bubbles more time to pop and disappear. Pour it into a paint tray.
Apply the second coat of polyurethane, allow to dry and sand following the same method as the first coat. Apply a third coat and don't sand it. Allow the final coat to dry for 24 hours before walking on it. Wait 72 hours before replacing furniture and 1 week before using rugs or cleaning the floor.