folded paperboard food cartons having a polymer coating only on the interior surface and sales graphics on a substantially unplasticized clay coated exterior surface are heat sealed at contiguous areas of overlapping flaps wherein the polymer coated interior surface is in direct contact with a small portion of the clay coated exterior surface or , where it is desired to secure together two portions of the clay coated exterior surface . in such heat sealing areas , small and accurately located exterior clay coated carton surface areas are primed with a water based polymer emulsion such as ethylene vinyl acetate that is precisely applied to such areas by a printing press .