To be honest I don't think keeping an eye on it will make any difference - my wheel arch liner is on perfectly. I think its just a design flaw.

The dealer told me they were starting to get a lot of mk5's coming in with this problem and that the cause is due to vibrations causing the liner to rub against the inside of the arch. Can't see it until its too late ...
For me its put the final nail in the coffin of buying a mk6 or any other VW. Compared to the mk4 build quality has gone downhill, and I can live with flimsier interior trim but rust on any 5 year old car, let alone a quasi premium brand like VW is just a joke.
Will be 5 years or so till its a problem for you guys
