Hi All
If rust breaks out from a previous repair, providing it was undertaken by a MB approved repairer, MB will authorize a second repair. I know, it happened to me. However, I still had to go through the authorization process with DC and it was 8 months before the work was done during which time the corrosion spread.
If it's surface rust, the dealer will want to examine the rust spots with a powerful magnifying glass and a paint thickness gauge. In the vast majority of cases they will claim the spots are untreated stone chips and therfore the owners responsibility. What I find curious is that stone chips can appear in some very strange, out-of-the-way places. Be prepared to hold your ground and get a second opinion. Dealers aren't going to part with money if they can help it.
Has any member used the paint touch-up kits you can buy in Halfords, etc? There's 3 pencil-like containers (rust inhibitor, undercoat, top coat?) in a clear plastic container resembling a cigar case. I believe there many be emery paper and other items in the kit.
REGARDS Phil