The container sides leave a welding machine in a row and the coating material is supplied at the side of the welding machine through a supply line (12) where it is sprayed on the inside evenly over the entire area onto the inner wall using a rotary spray element. Reverse suction is then carried out inside the container and the sprayed coating material is returned through the return line (13) at the side of the welding machine. The mutual spacing between the successive container sides is reduced in the coating area eg through a brake unit.