I've run across a few pieces that were "coated" the way Rick described. I've always suspected that some "well meaning soul" actually painted the piece right over the layer of GUNK. I further believe that at some point after painting, some OTHER "well meaning soul" heated the piece. Thus turning the gunk and paint into some type of indestructible substance that NASA and the US military should be made aware of... LOL
All jokes aside, I do believe that paint on top of the gunk is what I've encountered. And, I really do believe that it had been heated after painting. Electro doesn't do much to this stuff. Wire wheel and razor blades and LOTS of patience is all that's worked for me. But, I've never gone with a chemical removal route... might work.
Chuck, you let us know if that paint stripper works.