An epoxy vinyl paint and an epoxy vinyl varnish taken as free-standing films or applied to steel with different surface treatments were investigated by electrochemical impedance spectroscopy (EIS). Visual observations showed that the corrosion performance of the paint was better than that of the varnish. The correlation between protection effectiveness and EIS results was poor. This apparent contradiction is explained on the basis of a new model taking account of both the bulk properties of the film and the specific interactions between substrate and coating at the interface. An original experimental procedure is devised to separate the impedance of the inner layer of a coating in contact with the substrate from that of the outer layer in contact with the solution. Both varnish and paint coatings were analysed with this new method and revealed marked differences between the behaviours of the outer and inner layers. The adhesion range of the film on the substrate seems to contribute by a major part to its protective properties.