Hmm. Wonder if it works on sails,..

Rust Rust can be removed with a thin paste of baking soda or toothpaste and a soft-bristled toothbrush. Wash the area well with mild detergent and rinse thoroughly. This will not remove any stain embedded in the coating, but it will remove most of the surface stain. Remember, that using any abrasive on a sail reduces its life. For removing small areas of rust stain, a mild gel product named Magica is recommended.

Actually, any type of acid will dissolve rust including lemon juice. Most sails made of Dacron or Polyester are resistant to acids. The lighter acids take longer but still work. And washing the treated area with plenty of fresh water is important so as not to affect other items like sail covers.

I have used nitric acid which takes it out in seconds but one must know the precautionary measures. Carpet cleaning supplies carry a small bottle of Rust Remover that works real good. But Rubber gloves and safety glasses are a must when using any toxic chemicals.

I would use a solution of oxalic acid.
Bar Keepers Friend contain it with other ingredients but I would try it alone for a really tough rust stain.
It is a reducing agent and also works by chelating the iron.
It shouldn't affect a dacron sail.
Be aware of its toxic properties though.
You can buy it on eBay.