How to clean rusty spots on the sail?

[romandesign]

Last season when I raised my mast, some rusty water splayed out from the sail groove. It must have accumulated during winter from something that rusted somewhere. I though it was gone, but after I raised the sail, it splattered again, right on my white mainsail, leaving nasty rusty stains. How can I clean it? What I can do? The sail is otherwise in very decent condition, it's a shame to have those stains there...