Metal enclosure insulating paint/spray
Glyptal

It was invented by GE for literally that purpose. It's for painting metal parts of electrical motors, rotating equipment and cabinets.   

Aside from superlative electrical insulation, it has good thermal performance and low flammability.   This has resulted in off-label use to seal engine blocks. 

Don't paint parts of the panel that are not meant to be painted. It may be most expedient to protect the bus assembly by pulling it out entirely for painting.