You will need the following items:
• Oxalic acid
• 220 grit sandpaper
• 2 old toothbrushes
• White vinegar
• Pre-Stain wood conditioner
• Favorite sealant (We recommend Varathane)
What to Do
1. Sand the wood down with 220 grit sandpaper until smooth to touch. Wipe the dust off using a damp paper towel.
2. In a glass container, mix 1 tablespoon of oxalic acid with two cups of warm water. Dip an old tooth brush into the solution and tap the brush directly over the stain(s), allowing a small puddle to form on the wood. Let it stand for 15 minutes or so.
3. Mix 1 tablespoon of distilled white vinegar with 2 cups of warm water. Take the second toothbrush, and dip it into the vinegar solution and spread it over the area where the oxalic acid was applied. This will neutralize the acid.
4. Allow the wood to dry completely.
5. Repeat the above procedure until all traces of the stain(s) have been removed. Twice is usually enough.
6. When the stain(s) have gone and the wood is completely dry, apply the Pre-Stain wood conditioner as directed by the instructions on the can. When the conditioner has dried completely, apply your favorite sealant as directed by instructions provided.
N.B. This should be done outside, or in a well-ventilated area while wearing protective gloves. Oxalic acid is toxic and can irritate the skin.
Source: hometalk
