Charging electric vehicles (EVs) wirelessly, while in motion, is considered a key milestone in achieving sustainable and uninterrupted EV mobility. Dynamic underground wireless power transmission (WPT) systems offer promising solutions to this challenge. Once installed, these systems must withstand harsh underground conditions, including elevated temperatures, water ingress, and mechanical impacts resulting from above-ground operations. To protect WPT components, civil-grade epoxy materials are commonly used as protective barriers. This study investigates the long-term durability of two commercially available epoxy coating materials for underground WPT systems, combining both experimental testing and numerical modeling. Water diffusion tests are conducted at room (23 °C) and high (50 °C) temperatures to examine the temperature-dependent diffusion characteristics of each epoxy material. A finite element (FE) model is developed to simulate the progressive damage caused by water diffusion under thermal and mechanical loadings. The numerical investigation involves sequentially coupled analyses of heat transfer, water diffusion, swelling due to water uptake, and low-velocity impact. Results show good agreement with experimental findings, confirming the model’s capability to predict the performance and degradation of epoxy materials under actual service conditions.