Just took off a tenacious rust stain from the boat:
http://barkeepersfriend.com/products.htm
Read about it on another forum, saw some at Lowes, bought a can for < $3.00 and made a little paste (its like Comet without the grit), killed the stain in a few seconds.
"Atticus Recommended"
I have also been known to be a bar keeper's friend.
Bar Keeper's Friend
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