Durable and easy to maintain, oak is used to build a variety of items including tables, desks, chairs and flooring. Like other wood surfaces, oak is susceptible to stains, dust, dirt and grime. When water and other liquids are spilled on oak surfaces, the liquids dry and leave a black stain on the wood. This black area is a challenge to remove. You don't want to use a cleaner that can damage the wood or cause the stain to penetrate further into the wood.
Mix 450 g of wood bleach crystals and 4.5 litres of hot water in a bucket. Wear safety goggles, rubber gloves and an apron to keep the chemical out of your eyes and off your skin.
Stir the mixture with a scrubbing brush to dissolve the crystals.
Apply the oxalic acid solution to the oak wood surface using the scrubbing brush. You will notice the wood starting to lighten.
Scrub the oak wood surface thoroughly to remove the black stain. If the oxalic acid solution cools, add hot water to the mixture. Do not boil the solution on a hob or other heat source.
Continue applying the solution and scrubbing until the black stain is gone.
Wipe the oak wood surface dry with a clean, non-abrasive rag or towel.
Rinsing and sanding
Fill a bucket with 4.5 litres of hot water.
Dip a soft non-abrasive rag or towel in the bucket of water. Wring out excess water.
Rinse the oak wood surface with the damp rag or towel.
Repeat these steps two to three times.
Dry the oak wood surface with a clean non-abrasive rag or towel. Let the oak wood air dry overnight.
Sand the oak wood surface with a piece of 200-grit or higher sandpaper. The oak wood must be completely dry before sanding.
Sweep the oak wood with a non-abrasive rag or towel to remove any sand dust from the wood surface.
Use a soft paintbrush to apply a urethane floor finish to the oak wood surface, if desired.
Work in sections if the black area is large or the staining is deep.
Store any unused oxalic acid crystals in a closed container. Place the container in a cool, dry place.
Oxalic acid crystals can cause burns and might be fatal if swallowed. Keep the substance away from children and pets.