If the car is a nice one... I woud look at getting the job done properly, shot blast the rust, or cut out the rot and weld in new, save the roof first, thats the most difficult to save once the dreaded rot has gone too far! attacking with an angle grinder takes out too much balls from the steel, shot blasting is the way to go, it removes only the rust.. wire wheel brushes only touch the surface rust, they will NOT remove rust that has penetrated the surface of the metal, they only polish it!
Tom