i had a long chap with the guy who replaced my 16v arches and it the result of a bont behind the arch, the bit behind (about 2-3"s from the top of the arch is bonded behind the arch and often this causes a water trap behind the arch this is why the rust is so bad because i from the inside out and IMO there is not hing you can do, filler will lift within a year or two, one thing that might be true is that some a worse the others and indeed late ones didbt always suffer like this where thye had resolved it, My 1995 williams 3 has odd bits of rust here and there but not on the arch like my 1992 16v and almost all other 16vs i have seen.
IMO you have two choices -
- New rear quater - £700ish but should look perfect
- New arch only - £250ish which would have to be from a base model but the differnce is nothing but its far more difficult to get it cosmeticaly seemless be bacuse the weld run the length of the arch and is flat so very difficult filler 100% perfect (which is more time consuming and therefore expensive)
On the birght side the clio is genraly very good struturaly, loads of MK3 fiestas are condemedby this age due to MOT failing rust
Hope this helps