Ercol furniture seems to be sometimes/always stained with a horrible stain, yellow in the case of an elm coffee table I have. How do you remove it? I tried oxalic acid, but it did not remove it all after 2 or 3 applications. (And warping the table due to the water content, now flat due to applying water on the opposite side.) I've now stained and oiled that table, but for future reference I'd like to know how to remove it.