Please, for the love of anything in your life, don't use foam anything to fill the area. You will get water in there and the rust will come back.

Spray plenty of cavity wax in the gap and move on.

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Onus probandi incumbit ei qui dicit, non ei qui negat

I recondition w123/w126/w124/w140/r107/r129/ steering boxes!


1984 300D "Elsa" odo reset 6/2011 147k
1983 300TD "Mitzi" ~268k OM603 powered
1995 E300 "Adelheid" 262k [Sold]

No, no, and no again to foam. It traps water and makes any future repair painful.

3M makes a good cavity wax, but I believe it is most effective on clean metal (vs. already rusted); for surfaces with some corrosion an oil-based product is better - and I'm not smart / experienced enough to recommend one. Good read on the topic: https://bobistheoilguy.com/forums/threads/woolwax-fluid-film-wax-cavity-protection-which-is-best-for-an-old-car.340637/

And I'll go back to my prior post, where you can't weld, epoxy is an alternative.

Good luck with the repair!

It would be ok to use foam to make a form but ONLY if it can be removed afterwards. Leaving foam in will make the problem you're trying to fix worse.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Well, OK. I was thinking of a rubberized foam. They sell this at a rubber supply place that is local to me in different thicknesses. I have used it many times in other cars that were being fully restored by a shop I used to work for. I do not believe it has any absorbent properties.
I guess if it is being recommended against, I'll have to think of another way to go. I'm not sure how I'll be able to get the fiberglass patch to hold it's shape even if I use wire mesh.

I started making a template that will be used when I make the patch panel that will be installed.



This car in general is turning into a far bigger issue than I was hoping. I'm finding rust in all sorts of inconvenient locations.
One example of this is the sunroof tray. The rear corners are completely gone. I will need to remove the tray in order to deal with it.



I'm having to seriously consider whether all this is actually worth my time. I was hoping to have the car on the road by early Summer. But, that is getting further away from being possible. Every time I find something like this it takes time and money (and morale) away from dealing with the issues I already knew the car had.