All old cars have rust. We can remove the rust using the proper techniques, then make the necessary repairs and weld in new metal.
Here is a typical example of rust on the lower rear section of a 1966 Mustang quarter panel. The rust has been removed and a suitable pre-made patch trimmed and positioned into place for welding
This is the after of the picture to the left. If you look close you can see the panel has only been primed and there is no body filler (bondo). The weld has been simply ground flush. A repair like this is essentially seamless even when viewed from inside the truck.