An assembly is disclosed treating a moving web (1) of paper or paperboard with a web treatment substance, the assembly comprising an application chamber (P2) located in a close vicinity of the moving web (1) and facing the web (1), whereby the chamber is delineated by the web (1), the walls (12) of the coater apparatus and sealing means (18, 24, 35) adapted to operate at the web ingoing and outgoing sides of the application chamber (P2). For application of the web treatment substance, the assembly includes at least one linear nozzle array incorporating at least one nozzle (21) for spraying the web treatment substance onto the web (1). The coater apparatus is divided into chambers so that a suction chamber (P3) is located behind the application chamber (P2) as seen from the direction of the web (1) and is adapted to communicate with application chamber (P2), and a flow is established into the suction chamber (P3) and the application chamber (P2) for removing mist and accumulations of the web treatment substance in such a manner that the pressure of the application chamber (P2) is made lower than the ambient pressure and the pressure of the suction chamber (P3) lower than that of the application chamber. In the assembly, the free flow direction of substances contained in the assembly is firstly from the surroundings into the application chamber (P2) and then from the application chamber (P2) to the suction chamber (P3) and means (22) serving to establish the flow.