Organic coatings are widely used for the protection of structures against marine corrosion. Based on long-term service experience, their thickness reaches often as high as several hundreds of micrometers. However, for primer coatings, where the barrier effect is believed to play a minor protective purpose, the surface preparation and the nature of the substrate seem to play an important role. Experiments have been performed on free-standing and supported coatings and the comparison showed considerable differences. It is postulated that the adhesion mechanisms provide an important part of the protection afforded by the coating. The insertion of a conducting probe inside the coating allowed separating the impedance of the inner part of the coating in contact with the substrate from that of the outer part in contact with the solution. Information on the influence of the adhesion on the protection was then evidenced. A gold electrode, deposited on the surface of the inner half-layer of the coating, was used as a potential probe to perform electrochemical impedance measurements. The first results presented in this paper show a clear difference between the behaviour of these two parts of the coating.