A printing roller has an outer surface and is rotatable in a rotational sense about a roller axis. A liquid applicator has an upstream blade having an edge engageable with the roller surface and a downstream blade having an edge engageable with the roller surface downstream in the sense from the upstream-blade edge. The blades form a compartment fillable with a liquid to be applied to the roller outer surface. The upstream blade is shifted on rotation of the roller between an engaged position with the respective edge engaging the surface and a disengaged position with the respective edge spaced radially from the surface.