Dark spots on hardwood flooring are commonly the result of some type of liquid that has soaked into the polyurethane finish and top layers of wood. Unlike white water stains residing on the very surface of the hardwood floor, dark stains require more steps and products in order to restore the beauty of the wood. Removing these dark spots from your floor and restoring the hardwood will take a bit of patience and hard work.
Wear rubber gloves, long sleeves, safety goggles and a filter mask to protect skin and prevent breathing in the harsh fumes of the chemicals you will use.
Surround the dark spots on the hardwood floor with a frame of masking tape. Roll up thick towels to create a barrier all around the existing dark stain. Using the tape and towel barrier will protect the nondamaged parts of the hardwood floor.
Sprinkle two to four drops of white spirit -- depending upon the size of the spot -- over the dark spot in the hardwood floor. Rub the dark spot with a piece of #000 steel wool, alternatively resprinkling the spot with white spirit and rubbing with the steel wool until the polyurethane finish is removed.
Fill a bowl with white vinegar and dip a clean cloth into it. Lift the cloth from the vinegar and squeeze the excess out with your hands. Place the vinegar cloth on top of the dark stain, and allow it to sit for five minutes. If, after five minutes, you notice the vinegar beginning to lighten the dark stain within the hardwood floor, dampen the cloth with more vinegar and resoak the stain for five more minutes. Continue on to the proceeding steps if the vinegar had no effect on the dark spot.
Pour 1 qt. of hot -- but not boiling -- water into a glass bowl. Add 59.1 to 118ml. of oxalic acid to the hot water.
Dampen a paintbrush with the diluted oxalic acid and apply the solution to the dark spot. Allow the oxalic acid to sit on the dark spot until the stain lightens to match the surrounding hardwood floor.
Rinse the area with clean water up to three times to remove any remaining residue from the oxalic acid solution. Use a towel to dry the hardwood floor. Allow the wood to completely dry for two to four hours.
Sand the area with a piece of fine-grit sandpaper until it is smooth, and then use a hand-held vacuum to remove the sanding dust.
Apply matching wood stain onto the area with a clean cloth. Wipe the excess stain off the wood after at least two to five minutes. Continue to apply wood stain until the colour of the area matches the surrounding hardwood floor. Allow the wood stain to dry overnight.
Brush a thin coat of polyurethane finish onto the area with a foam paintbrush. Allow the finish coat to dry for at least four hours, and then sand the area with a piece of steel wool.
Remove the sanding dust with the hand-held vacuum, and brush an additional coat of polyurethane onto the area. Allow the finish to dry overnight.
Foam paintbrushes eliminate brush marks within the wet polyurethane finish and reduce the amount of bubbles that appear.
Oxalic acid is poisonous, so keep away from children and pets.