In sci.electronics.repair Sam Goldwasser wrote:

This type is usually dark gray and soft - about the consistency of a pencil
eraser, maybe a bit tougher. It can be removed laboriously with knives,
picks, and elbow grease. But the question is whether there is some easier
way to do this that would leave the underlying components undamaged.
This stuff is used in a variety of places including PCBs and laser
tube assemblies. Both of these are of interest to me. Modest heat
has no effect including immersing in boiling water - it's not hot-melt glue.


This probably isn't much help but we used to use a chemical called
Eccostrip 93 made by Emerson and Cuming (or cumming or cummings).

I don't know if it's sold anymore, probably on every EPA hazard chemical
list by now but I believe the main purpose of it was to attack something in
the resin.

It wasn't a miracle worker, but did it's job eventually. Generally you
needed to apply it with a glass eyedropper to keep the area wet, then
periodically using a acid brush, dust off the powder, reapply.

As far as component damage, yeah, some carbon film resistors, the bodies
would flake off but I don't remember anything else. Even ic's (chips) that I
thought had some epoxy holding them together were no worse for wear even
soaking in the stuff.

-bruce