As long as you KILL the rust first after sanding and wire wheeling to clean out pitted area's ,your fine. After I clean out pitted area's I then proceed to KILL rust using a red scotch brite pad and VINEGAR WATER MIX 3 parts water to 1 part WHITE VINEGAR scuffing hard into the pitted area for a minute. Let sit for 10 minutes come back and wipe residue off with clean fresh water and paper towel's. the rust will turn BLACK.You KILLED it and it will not grow back. Then use what ever you want to fill over with. This will protect and PRESERVE the car and paint job for another 20 years.
I agree,replacing panels is no fun and In my OPINION, takes away the car's original production BUILD. Alot of people replace all quarter's and door's and fender's then they say its an original survivor but we know it has been destroyed. I prefeer to PRESERVE the whole car if possible!
That mean's treating rusted area's (KILL IT) The car feels right you know.