Artworks are often coated with a protective polymer (i.e., paraloid) film to prevent physical and chemical damage, and this film must be removed and replaced regularly, as it becomes dirty and damaged over time. Nanostructured fluids (NSFs) are used to soften and swell the paraloids. However, quantitative evaluation methods for understanding the swelling and detachment of polymer coatings have not been fully established. Monitoring the H2O uptake of the protective coating of an artwork upon cleaning with an NSF should improve the safety of NSF application in restoration and conservation settings by providing the timing of the maximal H2O uptake. This is a useful indicator to avoid excessive NSF penetration and damage to the substrate or painting layer. In this study, a method of monitoring the levels of H2O uptake of paraloid films upon NSF exposure was developed, and this method could be used to objectively determine the optimal timing for physical removal. First, the proposed method, based on electrochemical impedance spectroscopy (EIS), was demonstrated using conventional trans-type measurement and paraloid films on Cu substrates, and a needle probe was then developed to enable the cis-type measurement of paraloid films on nonconducting substrates. Using this needle probe, the degrees of H2O uptake of paraloid films on glass and painted canvas were successfully monitored, and the optimal timings for physical removal were 106 and 147 min, respectively. The results provide a proof of concept for the cis-type EIS of films and the monitoring of the levels of H2O uptake of protective films on artworks during cleaning, which could contribute to more objective and reproducible cleaning of artworks.