Where is the coupler (high or low on the keg)? How are you heating the keg?
If the fitting is at the bottom and you use a jet engine to gas heat the thing, you may have issues. Having the flame hit the epoxy isn't a good thing.
Assuming that's not a limitation, grind back to clean metal and goop away. If it's on the outside of the keg, contact with wort is not a big issue. If you do it on the inside it really should be "food safe" (which it's not). It's been done before and people don't seem to die from it. I'd certainly let it cure for a couple weeks before running anything through the system.