My wifes 95 N 1.9D has done 185K in dismal Scottish weather and doesnt have any bubbling on the rear arches. But then either its luck or due to the fact that I always pressure wash or hose the rear inside arches when giving it a wash! Sediment trapped in or against the arch seams is like a damp sponge and never really dries out. A small crack or chip in the seam is all it needs to start the rotting process on its trail. Therefore, as already explained it eventually finds its way through the panel metal, galvanised or not! One bit that has bubbled badly on the wifes car is around the rear wiper on the tailgate. Incidently her collegue has a similar 95 N 1.9D 120K with bubbled rear arches and around the rear wiper also!
If the rust is not too widespread and only down to a few pin holes, the section can be sanded back (preferably bead blasted) gently peened in and leaded, as used by many classic restorers. I managed to do this on my old MK1 Escort rear arches with success (compared to previous Trustan 23/P40/P38 attempts) . The area is cleaned, gently heated, fluxed, lead bar melted onto panel, filed to profile, skim filled, sanded and painted. Tricky but produces a good finish. Also the tin/lead (solder) also provides parasitic protection to the neighbouring steel preventing further rusting, similar to galvanising. The seams around the repair inside and out should also be cleaned back and resealed with a good auto body sealing mastic and painted. Waxoyl applied to internal cavities/seams and inside of wings provide good protection when repair is complete in the future (never use underseal - if it separates from the panel it traps moisture!!).
If the rust is worse than above then a cut and repair panel will be required. The problem with this type of repair is retaining the original panel profile. Welding always causes some panel distortion and can promote more rust later on due to heat stress. Lots of filler is required to get things back in shape. Best left to the professionals.
Ultimate repair is obviously to replace the quarter panel. This is the most expensive, but if done professionally will outlast the rest of the car.
Sorry for being long-winded, but hope this helps!