Rust will always come back if you dont treat it properly.
Best way, and the way Im doing mine.. is cut both arches out, weld a new arch in (Normal Clio arch is used but cut to fit), smoothed then sprayed. Sealed underneath and if its a good job, it shouldnt come back!
Rust wont happen if theres no way in for the water, so getting it sealed and fully cut out is the only way.
Mines only surface rust one one side.. but for the extra bit more now, Ive chosen to get a new arch put on - dont want it coming back ever whilst I own it.
As for saying it will come back so why bother, well, it wont.. if they do the job properly, heh.
[Original post was something like that, heh]