There is disclosed a selective wetting apparatus comprising a roller rotatable about a roller axis and comprising a wettable roller surface; an applicator unit having a lip which extends parallel to the roller axis and is radially spaced apart from the roller surface by a gap, wherein the applicator unit is to convey liquid agent towards the roller so that the liquid agent forms a liquid bridge over the gap to wet a wetted axial portion of the roller surface; and a flow guide to direct a gas flow into a regulated axial portion of the gap to locally prevent formation of a liquid bridge, and thereby prevent wetting of a corresponding axial portion of the roller surface.