Just passing on what my dealer told me. They were very helpful, and explained it was a well known problem by VW. They said water and dirt gets into the gap between the lining and the arch, creating an abrasive paste that wears away at the arch and causes rust. They said normal moisture wouldn't affect the metal because it is fully galvanized, which is why they are confident enough to give such a long rust warranty. Again, i'm just passing on what they said, how accurate that is I wouldn't know.