An otherwise beautiful wooden floor can be ruined by the appearance of black nail marks. These marks are produced as the result of tannic acid in the wood coming into contact with iron and moisture. The best way to remove black nail marks from old wooden floors is to use oxalic acid. This type of acid works to remove the black stains in the wood, without harming the colour or structure of the wood itself.
Move any furniture or carpets that may be on the floor out of the way. If you have many black stains to remove, you may want to move all of the furniture out of the room.
Remove any finish from the floor from around the stains. Sand the floor or use a chemical finish remover. If using a chemical remover, wear the proper safety equipment and follow manufacturer's instructions. Open the windows in the room to ventilate it properly. If you will be sanding the floors, you can rent floor sanders from many hardware stores. Wear a mask and safety goggles to protect yourself from flying dust. You may also find that some of the black stains come out of the floor as you are sanding.
Mix the oxalic acid with hot water in the proportions recommended by the manufacturer. Apply the acid to the floor with a brush. Wear gloves to protect your hands and goggles to protect your eyes during this process. Allow the acid to dry overnight, and apply another coat, if needed. Repeat the application and drying process until the stains are removed.
Rinse the floor with distilled water to remove the oxalic acid. Apply two or three rinses to ensure that you have removed as much of the acid as possible.
Mix 2 tbsp of baking soda together with 1 qt. of water. Apply it over the wood floor to neutralise any remaining acid in the floor.
Allow the floor time to dry thoroughly. Reapply any protective varnish or lacquer.