the film forming composition may exist without support or reinforcement however in preferred form the film used to mask electronic circuits is a composite film in which a film substrate supports the film composition that provides the contact surface a primer layer may be interposed between the base film and the film forming composition materials suitable for use as substrates include polyethylene terephthalate polyimide polyphenylene sulfide fluorenone polyester and other films that remain essentially unchanged in contact with flexible circuit substrates at temperatures in range of 180 u00b0 c to 200 u00b0 c for periods of time up to about an hour films having thickness of from about 12 u03bcm to about 250 u03bcm are useful preferred substrates include pre shrunk polyethylene terephthalate film with a thickness of 25 u03bcm primers suitable for use on substrates of the invention include those having a rubber compound useful rubbery components include natural rubbers such as butyl rubbers and various synthetic compounds including but not limited to acrylonitrile butadiene acrylonitrile butadiene styrene copolymers styrene butadiene styrene styrene ethylene butylene styrene polychloroprene polybutadiene polyisoprene styrene isoprene styrene and mixtures thereof the primer may also contain an epoxy or a phenolic resin such as phenol formaldehyde resin available commercially from union carbide under the trade names ucar bkr 2620 and ucar ck 1635 novolak resins and the like and mixtures thereof the primer may further comprise additives such as tackifying agents antioxidants colorants viscosity adjusting agents.