In article ,
John Schmitt wrote:

http://www.envirosafemfg.com/rust_stain_remover.htm

is a possibility. You can also try oxalic acid. It is a white
crystalline powder which will dissolve in water or dilute
hydrochloric acid, which seems to speed things up. Hydrochloric
acid is corrosive and oxalic toxic. LD50 for oxalic acid would
appear to be about 25g, so accidentally swallowing a whole
ounce of the stuff would be unfortunate to say the least. You
may have to sign the poisons register at the pharmacy.

....... or buy a couple of pounds of Rhubarb and
boil it up.

--
Tony Williams.