Nick, I understood OK even before the editing, thanks. We must all be getting used to cell phone messages.
Eric, thanks for the link. That HV rating (2100 volts/mil) matches my MG Chemicals corona dope, so I should be OK with it too, I would think. I am applying a few coats of it in the area where the arc occurred now, after further cleaning of debris there.
Tomorrow, I am going to pick up a piece of glass-epoxy board for the top of the HV box (copper plated on one side, which can go toward the box's metal surface). It is rated for 40KV "dielectric breakdown" strength, which I hope is the property I should be looking at:
http://www.alliedelec.com/images/pro...S/70125831.pdf