Preventing Arc Discharge
Short for "Corona Dope," also found as high dielectric strength insulating paint.   Corona dope is still sold by the usual electronics chem suppliers.  I find that crazy glue cyanoacrylate works fine.   But note that these painted-on coatings are intended to stop HV leakage across a surface.   If you have sparks leaping across an air gap, then instead you need a thick material rather than paint.

Silicone caulk works fairly well, but use the ammonia-stink RTV type, not the water-base stuff found in paint stores.

Also note that arcing usually causes and is caused by a carbonized surface path.  Make sure to first scrape away any (possibly invisible) damaged insulating surface which has become conductive.