Abstract: An aqueous surface treatment bath that has a pH from 2.5 to 4.5 and contains at least 0.1 g/L of orthophosphate ions, from 0.1 to 3 g/L of condensed phosphate ions, from 0.1 to 0.5 g/L of free hydrofluonc acid, and at least 0.1 g/L of water-soluble polymer molecules that, except for short end groups as generally known in the art, conform to general formula (1) below: in which n is the degree of polymerization and has an integral value or each molecule, the average value of n for all of the molecules present being from 2 to 50 and not necessarily integral, X is a hydrogen atom or a moiety Y conforming to general formula (II) wherein each of R1 and R2 is an alkyl moiety or a hydroxy alkyl moiety having from 1 to 3 carbon atoms each, the total number of moieties Y having a ratio to the total number of aromatic rings that is from 0.2:1.0 to 1.0:1.