Previous academic studies and engineering practice have found that polymeric anticorrosion coatings are prone to premature failure in the deep ocean environment due to the accelerated water diffusion under high hydrostatic pressure. However, this observation is in contradiction with the free-volume-diffusion theory of polymer materials. This work aims to understand the abnormal rapid water diffusion mechanism by investigating the failure behavior of epoxy coatings containing glass flakes on Q235 mild steel. The samples were pre-processed in high-pressure by cyclic loading of 8.0 MPa in a dry autoclave. The results revealed that many micro cracks and voids around glass flake fillers, induced by high-pressure, provided fast pathways for water penetration, catastrophically accelerating the overall deterioration of the coating. The insights from this study is that micro cracks and voids around glass flake fillers induced by high pressure and the escape degree of residual gas in paint need to be seriously considered in the development of paints used for deep ocean anticorrosion application.