What's the group's experience with Armor All?
Seems to help non-porous surfaces, but I'd like something
that would help with the foam stuff that lines instrument cases.

That foam stuff isn't rubber, and it generally takes a long time (at least a
decade) to deteriorate.

Being porous, the foam's surface area is huge. You'd have to soak the foam
in the protectant! The protectant would wind up coating the equipment.

If there were some chemical that absorbed "bad chemicals" that promoted the
deterioration, it could be put within the case, in a porous bag.

I have several Pelican cases for photo and electronic equipment. They're
about 10 years old, and the foam shows no sign of deterioration. If it does
start disintegrating, I'll be able to buy a replacement insert (probably).