Water can wreak havoc on wood by leaving unsightly black stains on the surface. Simply placing a glass filled with a cold liquid on the wood surface for an extended time can create dark stains that are difficult to remove. A dark stain caused by water typically means the moisture has made its way through the wood's finish and soaked into the wood itself. Even though the black stains left behind from excess moisture do not prevent the wood from being functional, it will cause the item to appear old and dingy.
Fill a microwave-safe jar with 1 pint water and 2 to 4 tsp oxalic acid. Oxalic acid is also known as wood bleach and is available at home improvement centres.
Place the jar in the microwave and heat on high for 30-second intervals until the mixture is hot. This mixture must stay hot during the cleaning process. Reheat the contents in the microwave whenever it becomes cool.
Dip a paintbrush in the mixture and apply the bleach to the entire area. Brush the mixture on the wood in the direction of the wood grain. If you apply the bleach to just the black water spots, the area will have an uneven colour. Let the mixture sit on the wood for 10 to 20 minutes.
Examine the black water stain. Brush more mixture on the spot if it is still visible and let it sit for another 10 to 20 minutes.
Mix 1 qt. cool water with 1 tbsp ammonia. Dampen a white cloth in the mixture and wash the oxalic acid off the wood with the cloth. The ammonia mixture will neutralise the oxalic acid and stop the bleaching process.
Rinse the wood clean with a cloth dampened with cool water. Wipe the excess liquid off the wood with a towel. Let the wood dry for at least 24 hours.
The water and oxalic acid could raise the grain of the wood. If so, before restaining the wood, gently rub a piece of fine-grit sandpaper over the area to even out the wood grain.