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 (71,72,73) are primed with a water based polymer emulsion such as ethylene vinyl acetate that is precisely applied to such areas by a printing press.