My 2006 TFSI which I've owned from new, earlier this year, developed a rust blister at the bottom of the N/S front wing were it meets the sill, I pulled the arch liner out of the way and there was about 5cm of dirt accumulated there, I dug this out to see that under the muck it had rusted through. I called SEAT customer care and they told be to take it to a dealers for inspection. I went to SEAT Hatfield's approved body
shop, the guy looked at the blister and declared it to be a stone chip, when I asked how a stone is going to hit the side of the car he declined to comment and also declined to comment on the fact the inside of the wing was rusted more than the outside!
I contacted customer care again and told them I didn't accept his opinion and asked for an independent inspection, which they surprisingly agreed to, and said they would even pay for it whatever the result!
I had to use their approved independent inspector, pvwi2010.co.uk, the guy who came to inspect the wing, was a former manager of the paint and body
shop at the Vauxhall factory in Luton, he took one glance at the blister and said "it's not a stone chip". He carried out the routine inspection, measured average paint thickness and stuck a small spike through the blister in a couple of places and photographed it for the report, which stated that it had rusted from the inside due to the build up of dirt and long term exposure to road salts causing a failure of the paint and metal finishes.
End result, wing replaced and painted to full factory standards under the 12 year anti-perforation warranty.
Be warned: check your arches and clean any dirt accumulation away and treat with Dinitrol or Waxoyl to be safe.
BTW: the drivers side was OK.