[emeritus]

I have not had much practical experience with really high voltage stuff but from what I have read, flashover rarely happens inside solid bodies but at the solid/air interface. Failure of insulation in high voltage cables is usually initiated at voids within the insulation. The oil used in high voltage transformers and some high power high voltage cables not only helps dissipate heat, but also eliminates voids at which flashover might occur. So if you can ensure that there are no voids at the resistor/potting interface, you should be OK.

At Plesseys, the curing of the potting compound of high reliability stuff was often carried out in a vacuum chamber to facilitate removal of any small air bubbles that were created during mixing. The low pressure caused small bubbles that would otherwise have stayed put in suspension, to expand and rise to the surface. I used to heat up the Araldite I was using in order to shorten its (2 hrs at room temp) cure time, but I did always notice that, in its highly fluid warm state, any entrapped bubble of air rose to the surface and burst, resulting in void-free potting.