The impregnating material was selected for storage tank stability, low viscosity, and economic availability. Examples of these unsaturated reactive monomers, containing the group –C=C–, are styrene, vinyl styrene, methyl methacrylate, and a number of other monomers that can be used or mixed with the primary monomer to make up the impregnating composition. These compositions are fluids that will not readily gel or thermoset before coming in intimate contact with the polyester resin in the mica flake tape, even when they are exposed to moderate heating.