In a process for forming a multilayer coated film which comprises coating a multilayer coat-forming paint on the surface of a substrate to form a multilayer coated film thereon, the improvement wherein prior to the coating, the surface of the substrate is treated with a solution containing at least one onium compound selected from compounds of formulae (I) and (II) below ##STR1## wherein Y represents a nitrogen, phosphorus or arsenic atom, R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are identical or different and each represents a hydrogen atom or an organic group having not more than 8 carbon atoms, and X.sub..crclbar. represents an anion.