Bakelite Lacquer f.ex.:
 
  http://www.stekloplast.com.ua/eng/production/resin-lacquer/lbs-20/

with or without filler, Silica gel (Sodium Silicate) and mica, frit (glass 
powder) or fiberglass filler. Of the last two frit is less evil to handle and 
breathe. Bakelite lacquer is unbelievably toxic, smells really bad, does not 
keep and must be stored chilled (so shipping it in summer is a bad idea). The
oven where it will be cured must be flushed free of air and the exhaust dealt 
with. There are also more modern resins used for high temperature conductor 
encapsulation.

For something you want to do in a non-industrial setting silicone is probably
the only option.

Peter P.


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