A hardwood floor can be refinished by sanding the top layer of finish, then applying a new coat of finish. It is not always necessary to sand down to bare wood to refinish a floor. However, as Lowe's points out, it is important to determine how deep a refinishing job is needed before starting.
Before beginning a refinishing job on wood floors, the homeowner can test the wood by pouring a few drops of water on the floor. If the water beads up, the floor typically requires only cleaning and polishing rather than refinishing. If the water soaks in right away, the floor often requires refinishing. If the floor is warped or damaged, it must be repaired prior to any refinishing work.
Refinishing a floor involves removing the top layers of finish with a pole sander, steel wool or a floor buffer. If scratches on the floor are not deep, only a floor buffer is needed. The sander or buffer is used to rough up and remove the top layer of finish. The floor is then vacuumed, so all the dust and grit is removed.
Match and use the same wood finish that was used last on the floor. According to This Old House, a clear water-based polyurethane finish dries in a few hours, but an oil-based finish is often less expensive. This Old Home recommends that homeowners apply a new finish every two years, or when the floor starts to look worn or damaged.