Provided is a method for stiffening a curved sheet-metal panel (1), in which a flat cardboard panel (7) is applied to the concave face (1a) of the curved sheet-metal panel (1), characterized by the following steps: a) a number of parallel cuts (8) that pass partially through the thickness of the cardboard panel (7) are made in that face (7a) of the cardboard panel (7) that is intended to be applied to the concave face (1a) of the sheet-metal panel (1), b) a number of adhesive strips (9) that each cover said parallel cuts (8) are applied to said face (7a) of the cardboard panel (7), c) said face (7a), provided with said adhesive strips (9), of the cardboard panel is applied to said concave face (1a) of the sheet-metal panel (1).