The present invention relates to polymer based composition high voltage insulator having a monolithic coating of a one-part room temperature vulcanizable organopolysiloxane rubber composition which crosslinks in the presence of moisture. The one-part organopolysiloxane rubber composition comprises the product which is obtained by mixing from about 20 to about 60 weight percent of one or more polydiorganosiloxane fluids, from 0 to about 40 weight percent of a cyclo-organosiloxane, of the formula [(R)2SiO]n from 0 to about 40 weight percent of an inorganic extending or non-reinforcing filler, from about 0.5 to about 15 weight percent of an amorphous SiO2 reinforcing filler having a surface area of between about 100 to 250 m2/g and a particle size range between about 0.01 and 0.03 microns from about 1 to about 10 weight percent of an oximinosilane cross-linking agent.