Door rust... (Where is this coming from.)
 

I have an 81 300d. I'm currently working on getting the engine going, but in looking at it I have a lot of rust at the base of doors on the inside, below the door panel, and where the door meets frame along that seam. (Pulling up floor mat/rugs and searching around elsewhere I really don't see any rust. Looks like the frame has an undercoat also.)

The car came from an area where there was lots of water on the roads so I'm just curious about what failed that let it rust so bad there... :eek:

Things that I can see that could be a potential issue, yet none of them look bad enough to really cause this:
1: Windows seam's leaking and water sitting in the door.
2: Seam between door/Floorboard hard.

To me where the rust is seems a bit weird, as the very bottom edge of the door doesn't have any rust on it though.

(Is this a common problem that needs to be watched? )
Jp

I'd check two things, the window seals at the base of the window and the waterproofing membrane that separates the interior door panel from the inside of the door.

Its worse than you think
 

The rust is worse than you think - I promise. I just carried my '76 230 to the Benz graveyard three weeks ago due to a fatal case of rust. When I bought the car (W115 chassis), the rust didn't look too bad. A little on the rear fender and underneath the body was two little holes where the tires sling water up. The undercoating was original and very intact. When I got home with the car I pulled the rear seat to get the seat belts out and found more rust. The rust finally reared its ugly head when my entire rear suspension dropped down on the driver's side. Rust had eaten the mount up to the point that it no longer had anything to attach to! Fortunately, the suspension dropped when I was going 2mph across the parking lot at work, not when I was going 75mph down the expressway 10 minutes earlier. As I studied what happened, I found the area underneath the seat had fallen away with much metal entirely missing - it didn't look that bad when I got the car for sure. And the undercoating was completely intact. Treat the rust just like cancer - it can be fatal if not treated (in more ways than one - it could have killed me too).