A protective coating system for application to exposed surfaces of a transformer core prevents corrosion of the core. The protective coating is suitable for use in industrial and marine environments where many factors impact the life of the transformer core. The protective coating comprises at least three coating layers. The first coating layer is an inorganic zinc silicate primer. The second coating layer is a polysiloxane. The third coating layer is a room temperature or high temperature vulcanizing silicone rubber. A silicone rubber sealant may be further applied to outer edge surfaces of the core.