Get a can of WD-40 and one of those nylon/synthetic scratch pads that they make for teflon coated cookware, as it is less prone to damaging any surface finish....
It will take a lot of elbow effort, but worth it in the long run...
I don't know if they still produce it, but I think it was Outers that produced a product called "Rust Ridder" that was excellent for rust removal....Somewhere, I have a can in my cleaning box, just can't find it offhand.
A third choice would be some Liquid Wrench, but I'm not sure of the effect on any bluing/primary finish...
