Abstract: A conformal coating protects electrical components used in a high voltage high vacuum evironment from flashovers caused by patch charging. The coating comprises a semiconductor powder, preferably elemental boron, having a low atomic number and uniformly dispersed throughout an organic binder such an an epoxy. The coating's surface resistivity is made to be high enough so that the coating acts as an electrical insulator. On the other hand, the coating's surface resistivity is sufficiently low that any patch charge is siphoned off to the nearest conductor. The surface resistivity is regulated by the proportion of semiconductor present. The coating's secondary electron emission coefficient is 1 or just under 1. The coating has the properties of adhesion, stability, elasticity, provision of mechanical support, and resistance to solvents and heat. An example is given in which the epoxy comprises resin, fine particles of elemental boron, and a polyamide hardener.