I just moved into a home that has a 54'x28' gambrel style barn that has a metal roof. It's never been painted and has rust all over it. I would like to prime and paint it but can't find any available paints specificially for metal roofs? Is there special paints required? I'm looking for something in 'hunter green' style of color and that is durable...I don't want to have to repaint every few years.