Just put a new fuel filler gaiter on my M3 only to find rust on the inner wing behind the filler cap, fortunately it's only light surface rust and cleaned up OK, the problem is that the wheel arch liner has a 10mm gap either side and allows all sorts of muck to collect behind the liner, i added some rubber strips either side of the liner which looks like it should seal it OK. It's well worth checking especially if like me you live in the UK with it's crap climate !