Marty,
Before trying to paint I would try to stop the rust. A product that's good for stopping progression of rust is os-pho. My spelling might not be correct, but it's a pale-green liquid with about the consistency of water. I've bought it at Ace hardware in the past and they also have a "house brand" that's the same chemical. Follow the directions and it will provide a good base for painting.
I've seen primer marketed for use on galvanized metal that might work. Never used it.
I have used "kool-seal" and similar products designed to cover mobile home roofs with satisfactory results, but never on a galvanized roof. They seem to have an asphalt or tar base with fibers mixed in and dry to a silvery color. Generally sold in five-gallon cans. Hope this helps.
