Abstract: This invention relates to a high frequency heating process for treating a transformer core and windings. The process includes the steps of maintaining vacuum in an enclosure wherein the windings are to be treated, heating the windings with high frequency current, measuring the insulation resistance of the windings until a constant value is reached, stopping heating, filling the enclosure with oil, and then drying the oil from the enclosure. Alternatively, the transformer core and windings can be treated by encapsulating the winding with a polymerized resin, allowing the resin to flow into the mould and against the sides of the winding in a controlled fashion to enable polymerization of the resin to spread horizontally in a thin layer.