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I'm resisting a "magnets" joke in favor of real advice...
CLR. They sell it at hardware stores and such, it dissolves rust, calcium and lime deposits, nostly for bathroom cleaning but you can add a small amount to the launrdy and it works realy good for rust stains, and that's coming from a metal shop guy. No idea how it'll effect your waterproofness, but if you're wearing a white coat you probably don't care anyways.