107 Rust on firewall beam
 

I noticed some rust when looking up from passenger footwell. Two areas more or less under battery tray support. Checked on engine side and after scraping away some heavy undercoating, found that someone had done a repair of sorts. (this would have to have been before I bought car in 1989)

The one side is exposed and can be seen in picture below. The outboard hole and crack is not visible (even with light on underside). Measurent shows it is on other side of engine bay passenger side wall. Looking in there, I can see that there is some sort of bright metal patch in there, indicating some type of fix was attempted.

My question - How would they have accessed that area? Even with fender off, it looks like that area is enclosed between engine bay wall and inner fender.

https://dl.dropboxusercontent.com/u/...slfirewall.JPG
First two pictures taken looking up from footwell
https://dl.dropboxusercontent.com/u/...ewallrust0.jpg
https://dl.dropboxusercontent.com/u/...ll%20rust1.jpg
From engine side with and without patch
https://dl.dropboxusercontent.com/u/...ewallrust2.jpg
https://dl.dropboxusercontent.com/u/...LFIREWALL4.jpg

Engine out?

No engine is in. Doesn't interfere with much with possible weld repair except perhaps corner of beam near silver exhaust manifold in last picture.

My main question, is how to access the outboard rust, because it, by measurement, is mostly on other side of the engine bay side wall. If fender was taken off, that would expose the inner fender. I imagine it would also then have to be removed. Someone previously did some sort of a repair of the hidden part. Don't know how they got to it.

Don't know if construction is similar to W123s, but I should check.

I found this picture of a RHD 107. Other than for steering firewall beam is similar. Looks like a big job to expose the area behind the engine bay wall. Maybe have to cut wall away and then weld patch back?
https://dl.dropboxusercontent.com/u/...allexposed.jpg

I meant would it help if you removed the engine to repair this section - and do you think they perhaps did just that

No, I don't believe the engine was ever out. The patch they welded under the battery tray was easy enough to get at. Exhaust manifold is slightly in the way and it has been out 3 or 4 times :( (another story)

Part that puzzles me, is the repair that was done right where I added "Corrosion under here" in above picture. That area is totally enclosed. I have looked in there (through a hole) and there is a new looking metal plate welded in there. It actually looks like a reasonable job and likely not in a stressed area.

The engine bay corrosion needs treatment with POR or similar and patching ofholes. Would be difficult to do proper welds. So will likely rebuild with fibreglass, then reattach the metal plate I removed with pop-rivets or sheet metal screws. Then overcoat and seal entire patch.

The original repair was done well before I got the car - maybe 30 years ago, and it probably got no worse until I started exploring reason for those under dash holes!