The main question is how bad is it? If it's minor you could sand blast it or wire wheel it. If it has holes or is pitted bad then replacement/patching is the best method so it's structurally strong again.
__________________ 1968 Mustang coupe, Acapulco Blue, 289 2v, C-4, Power Steering.
Hopefully a 1969 convertible or Sportsroof (non Mach or Boss) for next Mustang project 
Trying to find my father's 1973 Mustang Grande he bought brand new. 3F04F126773 last known registration and title was in New Jersey, 1982. | 
frame rust on 72 project 
