Bare wooden floorboards are a popular look, but they need to be looked after. Liquids such as water spilled on floors, if not properly cleaned, can stain wood by permeating through the finish and causing black spots and marks to appear. This is also a problem with urine from pets or children. Spots can also be caused by mold or mildew if your home is not adequately ventilated. Removing these stains can seem difficult, but you just need the right materials.
Wipe away mold and mildew spots using a solution of one quarter cup of bleach mixed with one cup of water. If this doesn't work, the spots are probably water or urine damage. Apply mineral spirits to the affected area and rub with the steel wool pad. Allow the area to dry.
Soak a clean cloth or tea towel in vinegar and and gently rub the affected area and allow to dry.
Prepare a 50/50 solution of household bleach and water and rub the stain once again, if the previous measures have not proven effective. If this does not work you will have to use oxalic acid, a very strong bleach available from drug and hardware stores. Apply the acid directly to the spot, avoiding the surrounding wood. When the spot has gone add vinegar to neutralize the bleach.
Rinse the area with clean water, using another cloth, and then pat dry.When the area is perfectly dry, sand it down with a fine-grit sandpaper. It will be easier if you do this with a small, hand-held electric sander.
Wax the area with paste wax and buff it until it matches the rest of your floor. Do this by applying a thin layer of wax to the area and rubbing it forcefully, with a soft cloth, until the wax is gone.