With all the talk of rust removers, I'd thought I'd relay this info. 
I've had good luck with phosphoric acid (the stuff used in coca cola). 
It turns the rust into a more stable oxide and then you can prime/paint 
over it. I use this stuff by the drum in my buisiness. I pay about 
$0.50 a pound for it. I'd be glad to send it to anybody who needs it 
for my cost plus shipping. Hope this helps,

tony