you could used pool acid with a 3m red scuff pad use all things needed to work safe with acid rubber gloves face and lungs. wash spot down with alot of water and baking soda wash then with metal prep wipe dp epoxy it on it i do one spot at a time seal it up with a epoxy so it will not rust up or use a clean n strip wheel on a roof you do not want to sand blast it you will warp it all to hell