my 200d was a real POS when I found it. Paid $100.00 for it cause I liked the fins and the diesel. The thing did run, tranny shifted (auto) brakes worked. Had several holes in the rear quarter and rockers. All I wanted to do was make the car look ok with a decent paint job. Found the spray foam to be great for filling holes and since it is cured by water vapor it sucks the metal dry as it fills. After it cures it sands down very easy and takes glass great. Never had a repair fail or start to rust at the edges.
The only rust I ever found on my 300d was the spot under the battery tray. Looking through the posts here it seems to be a very common problem. The spot was about the size of a dime and whas where the water was getting in when the drains could not handle the amount of water from a big thunderstorm which can drop 2-4 inches of water in an hour or less almost every afternoon. Figured glass alone whould not displace any water inside this hole and besides if not backed up it may crack over time. So out comes a new can of foam. Pump that sucker full, wait for it to cure, glass over the top and paint. Since the tray covers it I didn't bother to paint. Leak was gone and the car has been dry ever since. I did replace the windshield seal about a month ago because it started to leak when going down the highway in the rain along the top edge. Bought the seal from fastlane, found a local shop that does all Mercedes warranty work here to install it for $50.00.
Driving to Naples tomorrow to pick up the 240. Will be kind of fun having a mini benz convoy of my very own on the way home.