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Back to Oxalic acid
here is a document published by a long term seller of Oxalic acid product.
quote from there.
Most finish removers that are used today containorganic solvents that don't darken the wood and since oxalic acid does not remove grayed wood fibers or organic discolorations like brown stain, blue stain or mold and mildew stains, what are the valid uses for Oxcon oxalic acid?1.Oxcon oxalic acid is the only product that removes inorganic discolorations of metal tannates and rust. If after stripping and cleaning the surface there are still dark colored streaks or blotches,there is a chance that a solution of Oxcon will remove them. This includes some types of “water marks.”2.If the stripping and cleaning process has turned the wood dark, a solution of Oxcon may help lighten it up
https://www.permachink.com/images/st...%20Acid-14.pdf
Good luck
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“Wood Or Paneling (Painted, Stained)
- To remove mildew from painted wooden surfaces, make a solution of 8-10 tablespoons of Washing (sal) Soda or Trisodium Phosphate (TSP) and 1 gallon of water. Scrub the mildewed area thoroughly using a soft-bristled brush. Rinse well with clear water, and allow the wood to dry thoroughly.
- If the mold has grown under the paint or varnish, remove all the paint or varnish from the stained areas. Scrub with a solution containing 8-10 tablespoons of Trisodium Phosphate, and 1 cup of chlorine bleach to 1 gallon of water. (Stronger solutions can be used if necessary.) Rinse thoroughly with clean water and allow to dry.
- If stain remains, apply oxalic acid (3 tablespoons to 1 pint of water). Finally, rinse the surface thoroughly with clear water. Dry completely before refinishing.
CAUTIONS:
- Oxalic acid is poisonous. Do not allow to touch your skin. Use rubber gloves.
- Be careful any time a commercial product or combination of cleaning supplies are used.
- Be sure to store and dispose of products/cans/containers as recommended. Keep out of reach of children.
- When using caustic, toxic, aromatic or flammable cleaning solutions, be certain to provide for adequate ventilation.
NOTE: Mildew-resistant paints are available in all colors for wood surfaces from paint and hardware stores. Manufacturers have formulated their products with fungicides to help combat mildew attachment and growth.”
https://www.fcs.uga.edu/extension/st...al-mildew-mold
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Good info re using washing soda