A method for stiffening a curved sheet-metal panel, in which a flat cardboard panel is applied to the concave face of the curved sheet-metal panel, includes a step in which a number of parallel cuts that pass partially through the thickness of the cardboard panel are made in that face of the cardboard panel that is intended to be applied to the concave face of the sheet-metal panel. A number of adhesive strips that each cover said parallel cuts are applied to said face of the cardboard panel and said face, provided with said adhesive strips, of the cardboard panel is applied to said concave face of the sheet-metal panel.