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Crazy… I nornally try with bicarbonate soda. A thick paste with water but not to thin. Soak in for 10 minutes. Rub in fir a while after… if needed repeat.
They are the worst stains to get out. Hope you manage.

Hm, the bicarb reacted with the oxalic acid and made it green, so we had to go over again with oxalic acid. Jamie's going to try a bleach solution tonight. I really don't want to replace boards. What a pain of a job!