Ionically permeable domains in polymeric protective coatings have been investigated empirically and theoretically. In the first part of the paper, results from recent experiments both complement and show agreement with received knowledge. This has allowed a physical model to be proposed for the mechanism of ionic conduction in polymeric coatings. This model assumes the random scattering of small regions with very different physical and chemical properties, akin almost to separate phase. Particularly the much higher rate of conduction is likely to have an impact on the coating's anti-corrosion ability. Based on the superimposition of permeable and impermeable domains, the model can be applied to allow comparison of multi-layer systems with single coat films of the same thickness. Such a statistical model has practical corollaries and in the second part of the paper statistical methods are advanced to allow determination of the probability of having these permeable domains in two, three, four and more coats. This has been further refined by a virtual simulation process using the distribution of permeable domains on a two dimensional plane. The significance of the theoretical models is then discussed with respect to the experimental data and what they mean in terms of protective ability.