Why won't BMW repair rust damage on my 2003 BMW M3 under warranty?

My (June) '03-registered BMW M3 has developed several small bubbles on the paintwork of the boot lid (on the rear-facing surface). These appear to be early areas of rust. One area is associated with a small scratch but the others are in "virgin" paintwork. The car is in very good condition and has been garaged for at least the last 18 months (since acquired by me). It has been fully serviced by a main dealer or independent BMW dealer. BMW have told me over the phone that it is now out of rust warranty (which was six years at the time of first registration, although this has been retrospectively extended for cars purchased a few months later, I gather). I am surprised that a 7-year-old BMW that has been well maintained is corroding in undamaged bodywork, and would value your advice on whether BMW have an obligation to repair, despite it being out of warranty.

Asked on 11 June 2010 by jorbell

Answered by Honest John
Is there any way you can view the underside of the area where this is occurring? If you can you will be able to see it it has been caused by moisture inside the boot causing rust on an unpainted or poorly painted section of the inside of the boot lid. But it's seven years old, so is out of the 6-year warranty.
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