wouldn't normal metal paint do?
I just did my metal gates last weekend (managed to get sunbrunt in the process ) and got the stuff, which you can use directly on rust, from Aldi, cheap as chips IIRC £3 for 2 tubs of 1kg each. Got lots left too, I'm sure one would be enough for my garage door.
Was a one off offer though, one of those weekly ones they do