The omnipresence of paints and adhesives in contact with metals requires a detailed knowledge of water diffusion and the ionic pathways through their polymer networks and along their interfaces. The E-coat, for example, generally shows uniform water diffusion. However, the substrate dissolves during film formation and enriches the E-coat. Iron ions in particular block the interfacial ion transport and normal water diffusion compared with zinc and aluminium ions. Moreover, Raman-SKP investigations show that the ions move through the E-coat in parallel to the aluminium interface. The pH-value immediately changes at the interface and the Volta potential increases with the pH-value, whereupon the interfacial dipole changes from ionic bonds to hydrogen bonds. Indeed, selective dealloyings or micro-galvanic couplings can cause the breakdown of the aluminium oxide and then the ion transport also takes place at the interface. Furthermore, various impedance approaches quantify the in-plane water and ion diffusion through various automotive polymer-metal systems. Concomitantly, the degree of plasticising, internal stress and topographical changes are assessed to clarify the core issue surrounding the Fickian and non-Fickian treatment. All in all, the methods and results developed offer several links for further paint and adhesive developments.