Oxalic acid definitely works. you can buy it from your local chemist.
fill bath with water, add a couple of desert spoons of oxalic acid. add sail. leave for an hour or so. drain & rinse with fresh water.
no rust stains.
homa
Please remember oxalic acid is very poisonous. It does not give off fumes and is not what is termed a strong acid so is not particulary dangerous to handle, but do not get it in your eyes, do not ingest any and keep it away from children.
Ha, Ha. Don't have iron topsail on this vessel, just japanese aluminium 4Hp egg whisk! Rust on sails has been inherited from previous owner, who must have put them away wet. They're foresails with s/steel luff rope, and I can only assume that's where the rust has come from. Anyway it looks bad, so I want it sorted.