As I tend to be a purest and have all the toys Migs Tigs and the like and use them daily, the best way is to cut out the affected area and weld in new fabricated "shaped " metal. As you have probally determined this requires both the equipment to form the metal hamers shot bag wheel ecc and the know how to use them. This method can be expensive. Alas if the rust is in fact surface and the structural integrity is not seriously affected there is a product on the market Call POR 15 and is like a phosphoric acid stuff that actually chemically bond with the Ferous oxide and encapslustes the rust. Best yet only approx $25.00 and brushes on!! Just "Google/Dogpile" or other search and find a local Dist. Uncle Gene