Heat registers

I have an oil burner and heat by hot water, and my heat registers in my rooms are looking pretty grungy, and somewhat rusty. Is it tough to replace those metal covers over the pipes, or should I attempt to paint them. And if I paint them, what should I use, since they get hot???
Thanks
Barry

Sand them down and paint them. Rust-Oleum works pretty good. Replacing them is no fun at all.