I need advice on best way to fix rust on the back lip of bed. I have a '63 with the two swing out doors (not ambulance) where the tailgate is. There is some pretty severe rust along that lip where the doors close onto the back. Is there a good solution to patching this? Have considered Bondo.  on't know if I would have to adhere some screen to the back side in order to fill the sizeable gaps in spots. Any suggestions would be appreciated.