Abstract: In a cardboard blank for a gas-tight and liquid-tight folding box, especially one printed externally, carrying on the cardboard surface which is to be the inside of the box a full-surface layer of heat-sealable plastic, the improvement which comprises an additional plastic layer positioned between the full-surface plastic layer and the cardboard, the additional layer being provided only over portions of the surface including the seam areas, the thickness of the full-surface plastic layer together with the local additional plastic layer being sufficient for tight, hot-sealed seams. The additional layer can be hot melt printed onto the cardboard, before application of the full-surface layer, at the same time the cardboard is printed and/or die cut. By providing the additional layer only in critical areas such as seams or fold lines, a thinner full-surface layer can be provided with an overall economy of plastic.