Good timing... I just did this yesterday and made a thread on peachparts
about it. Here's the content....
So I noticed about a year ago that I had a rust problem in the driver's side footwell of my 1984 300SD, when I pulled up my carpet and found this:
Needless to say, I did not want to let that stand. Unfortunately it took me a while to get around to it, but it didn't end up being as bad as I thought it would. In the meantime the attachment point to my accelerator pedal rusted through, but I doubt I would have been able to save that even if I'd fixed it immediately. For the last month or so i've been rolling just pressing the bar that the accelerator pedal used to attach to, I'll fix that at some point.
Anyhow, this was my strategy for fixing this-
1) Remove the pavement insulation/noise dampener that's currently in the footwell using a heat gun
2) Grind away the rust with my rotary tool (use Proxxon, not Dremel!)
3) Use some POR metal ready (I think this stuff is just phosphoric acid)
4) Paint it with POR-15
5) put down some new butyl rubber based sound dampener instead of pavement (from second skin audio, they call it 'damplifier' )
6) peace of mind!
It worked pretty well, and the rust wasn't nearly as bad as I had feared! I'm actually posting this while I'm waiting for the POR-15 to dry, by 10:30pm it will have been 4 hours, then I'm going to put on the butyl rubber.
Here are the pics!!
Here's after I pulled up as much pavement as I could with my hands, without using the heat gun-
Now putting up some foil so I don't inadvertently melt the pavement I don't want to mess with...
After most of it has been removed. Initially the pavement is composed of pieces the size of little pebbles. Using the heat gun melts them together into something that pulls off much more easily.
It stinks though, if I had to do it again though, I'd wear a mask while doing this part. I blew my nose after I was done, and it was pitch black!!! I bet it's like I just smoked two packs of cigarettes...
Ready to start grinding...
Partially done, my fantastic Proxxon rotary tool in the foreground. WAY tougher than dremel, it has a regulated motor (maintains speed under load), and uses a steel collet w/ chuck key.
Ready for painting. In the end, the rust was much less severe than I had anticipated. There's a divot in the floor pan, and before I had removed the asphalt I had thought it might have been a partially developed rust hole. Nothing but some surface rust, I was happy to be wrong!
Painted!!! Now I'm going to go back outside and see if it's dry...
I'm reasonably certain the A/C evaporator drain was the source of the water that caused this, as the one in there was totally useless. I've replaced it, and I strongly recommend that W126 owners at least have a look at theirs. It's pretty easy to check, you just need to pull the carpeted panel on the left away from the center console and it's right there.
Finished! Ended up using two layers of the butyl rubber stuff to match the thickness of the existing pavement insulation that I didn't cut away. Now I just need to fix the attachment point so I have a pedal...
Floor pan rust repair pics, or why to replace your A/C evap drain - PeachParts Mercedes ShopForum