A new simple experimental approach is proposed for in situ mechanistic studies of the underpaint stability of metal/oxide/polymer interface under immersion. It allows an intelligent screening and fast ranking of conversion coatings on Zn alloys designed for further application of organic coatings. The approach is based on the application of a simple electrochemical aging protocol to the samples preliminary coated with a model epoxy-polymer. The latter is enough thin to allow local electrochemical impedance measurement and stable under this protocol, as revealed by attenuated total reflectance infrared spectroscopy (ATR-IR) and electrochemical impedance spectroscopy (EIS). The interface stability and degradation modes characterization is accessed via combined EIS, local electrochemical impedance spectroscopy and mapping (LEIS and LEIM) and in situ optical imaging. The developed approach allowed to discriminate two Zn alloy substrates, one with intact and another with slightly damaged Cr(III) conversion layer, for which no strong differences in the electrochemical behavior or average surface composition was visible prior to the epoxy-polymer application. The relative stability of two substrates with model thin coating, evaluated in the developed electrochemical test, correlated with the observations obtained for the same substrates coated with thick commercial epoxy primer after 1000 h of immersion. The proposed methodology offers the possibility of a rapid intelligent screening of various formulations of Cr(III)-based surface treatments for Zn based substrates designed for paint applications.