There is described a method for varnishing security documents, especially intaglio-printed security documents such as banknotes, wherein both sides of the security documents are covered by a protective varnish. The method comprises the step of applying a thicker layer of protective varnish on a side of the security documents which exhibits a greater surface roughness, especially the side which is opposite to the side of the security documents which was last printed by intaglio printing. Also described in a varnishing machine for carrying out the above method.