Check trunk lid seam, under door seals and especially the car jack cavities on the rocker panel. I would remove the covers on these and spray the interior with Krown T-40 (or similar product). Rust can start here and eat into the rocker and underpanel. You generally won't notice it until it has progressed past the point of easy repair.

I also coated the inner rims of the wheel arches with a rubberized rockerguard paint, which really helped prevent rust there. Could use POR-15 as well.

Good luck!

__________________
Chris
2007 E550 4Matic - 61,000 Km - Iridium Silver, black leather, Sport package, Premium 2 package
2007 GL450 4Matic - 62,000 Km - Obsidian Black Metallic, black leather, all options
1998 E430 - sold
1989 300E - 333,000 Km - sold
1977 280E - sold
1971 250 - retired

"And a frign hat. They gave me a hat at the annual benefits meeting. I said. how does this benefit me. I dont have anything from the company.. So they gave me a hat." - TheDon