I was going to suggest Lexel around the rim, as it’s non-conductive. But it is flammable, being toluene-based. The high arc nature of this solenoid might not play well with any fumes trapped inside after it cured.
You’d think there’d be a rubber O ring, but from the pix apparently not. Not sure what one could use to seal it. Some solenoids, like are available ignition for our electrical bays, come enveloped in plastic, but how would one do that here ... even plastic dips may be flammable, although plastic epoxys spec sheets I checked don’t say so, and explosivity info isn’t given. Even Corrosion-X is flammable. I’d just worry about any product that needs to cure, even painted on the outside of the rim, and vapors being trapped inside the device.
Don’t know about something like Permatex Form-A-Gasket.
Joel
Hmmm... the product claims it’s sealed, so once disassembled that apparently was compromised:
https://www.intellitec.com/assets/pdf/1453-intellitec-pdf-template-53-00507-100.pdf