Semi-conductive electrical tape is commonly used for electrical field grading on high voltage splices; the idea being to create an equipotential field around your splice and minimize coronal losses. Same concept as corona rings on transmission lines.
We use Scotch #13, just enough to cover any sharp points or protruding surfaces, then over-wrap with insulating tape.
Corona! ....that's what it was!
I didn't know what it was then, and I don't know what it is now ...