In the present study, ex situ Fourier transform infrared (FTIR) chemical imaging and in situ electrochemical impedance spectroscopy (EIS) were combined to investigate the ageing process of a polyester/melamine coil-coated steel. The samples were first subjected to a QUV accelerated weathering test for 250 h up to 2000 h, followed by immersion in a 0.5 M NaCl solution to assess water uptake and polymer matrix plasticization. FTIR analyses revealed chemical degradation, including chain scission and the formation of polar groups, between 500 h and 2000 h of QUV exposure. Degradation effects were observed throughout the whole topcoat, with more significant degradation occurring near the surface. EIS measurements indicated greater water uptake with increasing QUV exposure, highlighting two regions of water sorption: an initial rapid Fickian diffusion region and a slower non-Fickian region. The time constant (τ) analysis, which was extracted from the EIS data and related to the dielectric manifestation of the glass transition, confirmed polymer matrix plasticization due to water uptake. Despite UV-induced degradation, the polymer maintained effective protective properties, as evidenced by the high low-frequency impedance unaffected by UV exposure or immersion duration (1 week). This methodology successfully identified ageing markers, providing a framework for studying UV degradation mechanisms, water uptake, and polymer mobility in anticorrosion coatings.