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Use a paint stirrer to thoroughly stir the polyurethane. The better you stir it, the less chance you have of ending up with bubbles on your floor. Don't shake the can of polyurethane, however, as this introduces more bubbles into the product. When you're finished stirring, pour the polyurethane into a paint tray. An 8 to 12 inch (20 to 31 cm) bristle brush is a good choice for this project. Avoid using lamb's wool pads, which collect dirt easily. Use long, even strokes to apply a thin coat of the polyurethane to the floor.  Avoid “flooding” the area with polyurethane -- you want a thin coat. Avoid going over the same spot multiple times, as this causes bubbles and imperfections. To avoid stepping on the surfaces you've already applied polyurethane to, it's important to start in the farthest corner from the entrance to the room and work your way toward the door. Aim to apply the polyurethane quickly, working from one end of the room to the other. Read the directions on the package to find out how long the polyurethane you're using  takes to dry. Typically, you'll be able to sand and apply another coat within 4 to 8 hours, though some varieties may take up to 12 hours to dry. Once the floor is dry, use 220-grit sandpaper to sand down imperfections. Make sure you sand along the lines of the natural grain, rather than against the grain. For smaller troublesome spots, use 320- or 400-grit sandpaper. Clean water-based polyurethane with mild soap and water. Use mineral spirits or paint thinner to clean oil-based polyurethane. Apply the cleaner with a soft cloth and wipe it dry when finished. For the second coat, you should thin the polyurethane so that it is less likely to create bubbles. Combine 10 parts polyurethane with 1 part mineral spirits (for oil-based polyurethane) or 1 part water (for water-based polyurethane) in a clean can and stir it thoroughly. Then, pour it into a paint tray. You will likely notice that some spots have a thicker layer of polyurethane on them than others. To ensure you have an even coat and seal, apply a thinned coat of polyurethane to the entire floor using the same method as before. Again, you'll need to wait until this coat is dry before moving on. Refer back to the instructions included with the polyurethane to determine the drying time. Don't rush this step, or your floor can get gummed up or ruined. Steel wool, grade 0000, works well for this. Use the fine steel wool to go over the entire surface of the floor, to remove imperfections as well as ensure the entire coating is even and level. Using the same method as before, wipe down the floor to remove dust or debris. Allow the floor to dry fully before adding the final coat of polyurethane. Use full-strength polyurethane for the final coat. Avoid cleaning or sanding this coat of polyurethane. Instead, stay out of the room until it has set completely. You shouldn't walk on the floor for 24 hours after applying the final coat. Don't replace your furniture for 72 hours and wait at least 1 week before cleaning the floor or arranging rugs on it.
Stir the polyurethane and pour it into a paint tray. Use a brush or painter's pad to apply a thin layer of polyurethane. Start at the farthest corner from the entrance to the room. Allow the coat to dry completely. Sand down bubbles or uneven patches. Wipe down the floor with a clean cloth. Thin the polyurethane and pour it into a paint tray. Apply a coat of thinned polyurethane. Allow the polyurethane to dry. Use an abrasive pad to even out the surface. Clean the floor to remove dust. Add a final coat of polyurethane. Let the final coat dry for 24 hours.