I had an annoying rust stain about the size of a silver dollar on my new mainsail. It came from being furled over an old block on my boom, and had penetrated right through the material. One day while out sailing I put a dab of Fibergalss Stain Remover, made by Davis, on either side of the sail and within about 15 minutes the stain had vanished .... and the sail didn't have a hole in it 
I've also used it on all my lower stain "less" steel fittings and it cleans them right up. Sometimes a second application is needed, but not always.
http://www.davisnet.com/product_documents/marine/manuals/790_FSR_INST.PDF
I just thought I'd pass that along .... all the usual disclaimers.
Peter
#1331 'Sin Tacha'
I've also used it on all my lower stain "less" steel fittings and it cleans them right up. Sometimes a second application is needed, but not always.
http://www.davisnet.com/product_documents/marine/manuals/790_FSR_INST.PDF
I just thought I'd pass that along .... all the usual disclaimers.
Peter
#1331 'Sin Tacha'