Few flooring options rival the natural beauty of a finished wood floor. Modern floor finishes like polyurethane are easy to use and act as a physical barrier against dirt and spills (see Reference 1). Finishing unfinished floorboards requires patience. Complete removal of dirt and sanding dust is necessary to produce a smooth finish. Each coat of finish must thoroughly dry before applying the next. Investing the time required to do the job right will produce results you'll take pride in for many years.
Wear a dust mask. Move the sander over the entire floor for each sandpaper grit. Start with the 60 grit and work your way up to 100. Coarser and finer grits may be used, depending on the floor condition. Keep the sander moving to prevent gouging.
Thoroughly vacuum, picking up as much wood dust as possible. Use a microfiber mop to pick up any remaining dust.
Wipe the floorboards with a slightly damp cloth to pick up excess dust. Immediately wipe the floorboards with a clean, dry cloth. Allow floorboards to air-dry thoroughly.
Brush on pre-stain wood conditioner if floorboards are softwood (pine, spruce, poplar) or hard-to-stain hardwood (maple, alder, aspen). After it soaks into the wood, wipe off any excess. Apply the stain of your choice within two hours.
Brush on the stain in the direction of the wood grain. After it soaks into the wood, wipe off any excess, in the direction of the grain. Check the product label for drying time and allow the floorboards to dry thoroughly. Use white spirit to clean brushes and any spilt stain.
Brush on a thin coat of polyurethane. Allow the maximum suggested drying time. Lightly sand the floorboards with fine sandpaper (220 grit). Remove all dust. Apply another thin coat of polyurethane, allowing 24 hours of drying time. More coats may be applied, lightly sanding and cleaning up thoroughly between each coat and allowing adequate drying time. Clean brush with white spirit.
If you prefer a more natural, traditional floor finish, try tung oil or linseed oil. Both these penetrating finishes have been used for more than a century. Although they're not as durable as modern finishes, they're easier to touch up (see Reference 2).
Ventilate rooms as much as possible while working and during drying times. Most floor refinishing products are flammable. Do not smoke or use flames in the work area until floorboards are completely dry.