Has anyone peeled back or cut into the yellow foam area that blends the fiberglass of the false firewall into the sheetmetal? In the first photo just to the immediate left of the yellow section, bordering it in the center of the photo looks like there could be rust where the glue attached to the metal (it matches the color of the obvious rust to the left), or could it be factory glue from anyone's experience? Just above it, if you enlarge you'll see a black strip of the false firewall foam that looks like a tenuous connection, and a little rust-color peeking through (or maybe old glue). If it's rust I can't just put miracle paint over the crusty old glue that's not secure. If you enlarge the photo, you'll see the peeling paint is right at the border, which could make matters worse. There also is a hex fastener in the lower corner of this area--I still think it looks like the area is glued down and if I unscrew the bolt, I don't want to crack the insulation. My concern is I could make it worse by probing into this area. If it's like a panel that could come off/back on, it wouldn't be too bad but it looks like the yellow foam might have an orangey-red glue at the sheetmetal which I'm not keen on violating.

I've already started remediating nearby rusty sections in the engine bay (while the engine and transmission are out for rebuild). I am grinding down to bare metal, then trading with phosphoric acid, neutralizing with acetone (Eastwood Pre-paint prep), then Miracle Paint. Then, I will feather in primer and matching color coat, and use seam sealer where applicable for originality under the paint (but on top of the Miracle Paint). Even thought it won't be seen, I want it to look like it was never really messed with.

That doesn't look OEM. Can you take another picture farther back?

Could there have been a previous repair done?

__________________
___________________________________________
2010 Toyota matrix

'93 500 SEL
A bad addiction. Takes all of my cash.

'12 Volvo S80 T6
Needed something that wasn't as hard to deal with as my bad addiction

'18 Mazda Miata
No more boring cars for everyday transport!

I found the same thing when I scrapped the sheetmetal on my 84 SD. It had some thick sealer over the aluminum plate above the bell housing on the firewall. The fiberglass was present too.

Some of the anti moisture putty MB used between seams looks like rust. I found that on the 85 SE body I'm working to restore now.

__________________
84 300SD
85 380SE
83 528e
95 318ic