A primer coating is provided for use on polymeric substrates to enhance the adhesion of metallized coatings to the substrates. The primer coating also improves the gas barrier properties of the substrate when used in combination with a metallized coating. The primer coating includes an amorphous polyvinyl alcohol, polyethyleneimine, and optionally, an aqueous dispersion of polyurethane. The primer coating may be applied to a variety of polymeric substrates including polylactic acid, polyethylene terephthalate, biaxially oriented polyethylene terephthalate, oriented polypropylene, biaxially oriented polypropylene, and biaxially oriented polyamide. When used in combination with the subsequently applied metallized coating, an oxygen transmission rate of about 0.1 and 5.0 cc/m2/day at 0% relative humidity is achieved.