This work investigates the properties of insulating coatings deposited by HVOF and atmospheric plasma spraying and the effects of various sealants. It is shown that adhesive tensile strength depends strongly on surface roughness and that the dielectric strength of a material is normally higher in the unsealed state. In the case of a new sealant, however, the dielectric strength of HVOF sprayed alumina coatings is shown to be higher in the sealed state, which potentially opens new applications in high-performance electronics.