Q7 wheel arches did exactly the same within the first 3 years, often contributed to by a lack of maintenance and care by owners neglecting to clean the affected area, I know because I learned from that very mistake.

If you have the opportunity to look after the arches from new, they maybe they have a better chance of avoiding corrosion?

If like me you don't use the rear seats a lot it's worth considering applying a light smear of clear grease on the outer painted surfaces of the rear wheel arches and renew as necessary every now and again, particularly in winter.
Combined with a liberal coating of grease to the rear surfaces this should ensure your rear arches stay in good condition.
Shouldn't be necessary though, they should be adequately protected at manufacture.

If you have the opportunity to look after the arches from new, they maybe they have a better chance of avoiding corrosion?[/quote]

That is fair comment. With offroading over the winter mud and salt will accumulate behind the wheel arch lip and I cannot always clean it all off quickly. However, my 2004 322 was probably cleaned less frequently than the 2012 car and it didn't rust within the 3 year paintwork warranty period (I think it lasted 5/6 years anyway). The 2004 car was waxoyled underneath from new, the 2012 car when I got it (after its first winter) and that might have made a difference. Only Range Rovers since 1988

Fortunately, our 2012 wheel arches are like new and so I've just spent an hour removing the original LR rubber strips (that have a metal core beneath the rubber); and I've degreased/ Iron X, then heavily greased the nooks and crannies around the arch and on the underside of the arch face before replacing the old LR rubber strips with the new plastic trims; all pretty much as recommended by NDF in the below thread and as successfully used on our last RR.

IMO, at £2.99 for one roll of plastic strip which would do the full rear arches twice times, this trim is better than the manufacturer's rubber because it doesn't have metal in it and unlike the LR ones which can go out of shape when you remove and refit them, these fit snugly on the arch. They protect the perfect width and they don't go out of shape.

http://www.fullfatrr.com/forum/topic22530.html?highlight=flange