I have tried the following:
- washing with soap & water,
- washing with dishwashing detergent,
- quick spray of white vinegar* (didn't want to leave vinegar on long in case it made things worse).
I tried Googling for ideas and kept coming up with just one: Oxalic Acid (the acid in rhubarb), which is used both to remove tannin &/or to bleach wood, apparently.
e.g. Video: http://www.youtube.com/watch?v=YTHiqtdZMMI
http://oxalicacidinfo.com/
Oxalic acid is inherently a strong acid: it is about 3,000 times stronger than acetic acid, which is the chemical name for the acid in ordinary vinegar... Oxalic acid is so strong that it is widely used industrially for bleaching and heavy-duty cleaning, notably for rust removal. If oxalic acid is not heavily diluted--as it is in plants--it is quite dangerous to humans, being both toxic and corrosive.
I am loath to use strong chemicals. Might vinegar*, if left on longer, or sunlight work instead? Anybody used Oxalic acid or have an opinion on it - is it reasonably "mild"?
*Apparently vinegar & iron are sometimes reacted together and then applied to wood as a stain/"ebonizer".