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Most home improvement stores will rent the equipment you need to refinish your floor and show you how to use it. For the big areas, you can use either a drum sander or a belt sander, which is typically a little easier to use. You also need an edge sander to help you get against walls and into corners. You’ll also need a floor buffer for finishing.
The sanding process will consist of 3 steps. Step one is to remove the old finish using a 36 grit belt in the sander. Do not sand completely down to the bare wood. Once the first layer is done, switch to an 80 grit paper and go over the floor a second time to remove any remaining finish or blemishes left on the wood. Finally, a third pass should be made with 100 grit paper to remove any scratches and prepare the wood for buffing.
After you finish buffing, let the dust settle for about 45 minutes and then vacuum the room thoroughly. If you’re going to apply an oil-based polyurethane, go over the floor with a tack cloth. If you’re applying a water-based finish, go over the floor with a lint-free rag that’s damp with alcohol instead.
To spread the polyurethane finish, you need a floor applicator and a 4 inch paintbrush. Work as if you were applying paint, keeping a wet edge so each stroke will blend easily into the next. Starting at one wall and always work the pad parallel to the grain direction. As you complete the stroke, lift the pad and pull it back with the grain. Keep working across the room in the same pattern until you get to the other side of the room, feathering the finish where it meets the wall. Allow 12 hours of drying time for oil based polyurethane and 2 hours for water-based.
Once the first coat is dry, sand it smooth with the floor buffer. If the finish has fully dried, the buffer will leave a powdery residue behind. This step dulls the finish before you apply the next coat. Vacuum the floor, then apply the second and then the final coat in the same way you applied the first. (Sand only between the first and second coats.)