When metal rusts, you'll find loose iron oxide flakes on the surface. The first thing you need to do is remove these, and it's usually fairly easy. Start by brushing off the flakes with a wire brush.

My roommate has an older Chevy Blazer (late 90's model year? Early 2000's?) and the roof is peeling paint badly. It's also rusting, so she'd like to get it painted. If she only wants to paint the roof ...