Sail Stain

I have a gray metal stain along the lower boom edge of my sail. what is the best method of preventing this.? This is a very hard stain to remove. Is there a product out there that will remove it?

There is a product called 'whink' or wink that is rust remover that might remove this stain..we have used it on other metal and rust stains on sails.
bob pattison
neilpryde sails int