Repairing a Rusty Roof?

We a problem with the metal roof on our church, rust. Every few years we have the costly job of repainting it. We are thinking of using, Extend, but have some questions. Is Extend recommended for use on metal roofs that see direct sun all day? Can you paint over Extend with a lighter color? How many years can we expect Extend to hold up in this application, before rust shows up again? What is the square footage coverage per gallon and the cost of a gallon? And finally, how many coats of Extend will it take to do the job right?

By David D.