A solid propellant rocket motor casing fabricated from strong filament in a matrix of curable polymer known in the trade as a composite motor casing is protected against permeation therethrough over long periods of time of water vapor from the atmosphere by a thin, lightweight, metal foil barrier comprising a metal foil adhesively backed tape such as aluminum foil tape spirally wrapped over the cylindrical or regular surface areas and by a separate, lightweight, metallic shell or dome having a thickness of 0.005 to 0.008 inches adhesively bonded onto the dome ends of the composite material cased solid propellant rocket motor, each metallic shell or dome being made by the use of a plastic or other destructible mold which is first coated with metal by an ion-vapor deposition process to a thickness of less than 0.005 inches, the thickness of the coating being subsequently increased by an electro-chemical coating process after which the mold is separated from the metal shell which then is adhesively bonded to the loaded rocket motor composite casing.