Mg-rich primers (MRPs) provide cathodic and barrier-type protection to aluminum substrates similar to that provided by zinc-rich primers (ZRPs) for steel. The former protection is driven by the more active Mg particles compared to the Al substrate, and the latter by the insulation of the substrate from the environment by Mg-containing products. A transmission line model was used to interpret ZRP electrochemical impedance spectroscopy (EIS) data that account for the contact impedance between zinc particles, the impedance associated with zinc dissolution, and the electrolyte resistance. Experiments were conducted for the evolution of the electrochemical behavior of an MRP on AA 2024-T3 under immersion in diluted Harrison's solution. The transmission line model was applicable to the MRP data. Comparison between the parameters of a ZRP and those for the MRP indicated that the loss of cathodic protection for an MRP was related to the interfacial resistance of the particles.