Abstract: A method for drying a thick wet layer coated on one face of a cardboard sheet, the method being, of the type in which the coated cardboard sheet is fed continuously and a high frequency electric field is applied thereto in such manner that the lines of force of said electric field go only through said thick layer to selectively heat said layer, wherein a further electric field is applied to the coated cardboard sheet in such manner that the lines of force thereof go at least through the cardboard sheet. The method includes starting to apply the further electric field before the first mentioned electric field is applied. Apparatus for performing the method is additionally provided, the apparatus including at least one pair of oppositely polarized electrodes positioned along one face of the cardboard sheet and a second pair of oppositely polarized electrodes positioned along the opposed face of the cardboard sheet.