A one-pack paint composition comprising polyester-melamine enamels and a predispersed amount of either phosphoric acid (H3PO4, in situ phosphatizing reagent), p-toluenesulfonic acid (p-TSA, polymer catalyst), or hydrofluoric acid (HF, surface etching agent) was formulated, coated, and thermally cured on a cold-rolled steel panel. The chemical natures of coating surface/interface interface were investigated by SEM-EDS and FTIR techniques, and the coating performance was verified by EIS measurements. The formation of metal phosphate layer and covalent bonding of P-O-C linkage at the coating/substrate interface was evident only for the unicoat system formulated with H3PO4 The single-step phosphate/paint system is shown to behave like a pure capacitance with excellent coating adhesion. The results indicate that the preadmixed H3PO4 in paint system does not simply function to activate the metal substrate like HF. Also, K3PO4 is not simply a function of the acid-catalyzed cure of a coating such as p-TSA. The in situ phosphatizing reagent, H3PO4 in coating formulation, however, tends to diffuse to and react "simultaneously" with metal substrate and polymer resin and thus provides superior protection of metal surfaces.