We investigated the impact of extended high-temperature and water exposure on fusion bonded epoxy (FBE) coatings applied to steel panels, commonly used in the oil and gas industry. We assessed the coating’s performance over an 85-week period at 65 °C in deionized water. Signs of degradation due to permeant sorption were evident within just 8 weeks. Cross-sectional analyses and focused ion beam milling revealed oxidation beneath the coating after 182 days, identifying the initial 200 days as critical for the onset of corrosion. This period likely signifies the beginning of the epoxy network’s breakdown and the establishment of a stable state of mass transport within the coating. Adhesion tests indicated a reduction in the coating’s pull-off strength due to water-induced plasticization, followed by a slight recovery attributed to secondary cross-linking. This research highlights the complexities in forecasting the failure times of epoxy coatings but provides vital data for constructing models to predict their lifespan under harsh conditions.