A method for operating a machine for single or double sided application of a liquid or viscous coating medium by way of at least one applicator unit onto the surface of a mating material web, especially a paper or cardboard web is provided. The coating medium collecting unit is cleaned prior to, during or after an application process of the machine, by way of supplying cleaning liquid via the supply line. The valve arrangement can be switched to the second valve condition for clean-up and to the third valve condition for the collection of coating medium in the medium collecting device. The medium collecting unit can be cleaned after start of the application process, during coating, through elements of supplying cleaning liquid via the supply line.