The invention relates to a coating mask (1) for electrolytically coating the piston ring groove (39) of a piston (38), which is made of an elastically deformable material and has openings (3 to 10) that are arranged axially and are distributed in a uniform manner over the periphery, into which rods (11 to 18) of an expansion device (19) can be introduced, the rods being arranged in a displaceable manner such that the expansion device (19) can increase the radial diameter of the coating mask (1) and also the inner opening (2) so that the piston (38) can be introduced into the inner opening (2). The radial diameter of the coating mask (1) is selected in such a manner that after the reduction of radial diameter of the coating mask (1) and the inner opening, the elastically tensed coating mask (1) presses sealing lips (44, 45) of the coating groove (37) against the piston (38), on both sides of the piston ring groove (39).