Whoops...I should have mentioned that it removes the patina...thanks sweetcostarica....
One more method that is as cheap as they come, though I found that it is more work than it is worth: lemon juice and salt. I know first hand the corrosive effects of salt, my Macbook Pro looks like it received shotgun treatment (my sweaty hands over time ate the finish in small areas of my computer) from me. So I'm pretty sure it works...but then, when I was attacking my deceased grandfather's father's old Vixen file with lemon/salt and an old toothbrush, (the file actually worked, and still works!) I was getting some rust off, but the rest of it was, for sure, a big part of the steel itself. So...what I am trying to say is...I don't know for sure if it works WELL, but it works if you want to try it.
Peace,
David