This study investigates the effect of paint-related properties on the accuracy of delamination detection in lined oil paintings using pulsed phase thermography (PPT). Mock-ups of lined oil paintings were examined by PPT under both normal and angled illumination to induce apparently localized heating. Spectral characteristics in the excitation and detection wavelength ranges were analyzed and related to phase contrast variations in the resulting images. While paint-dependent energy absorption under localized heating may blur phase contrast and lead to misidentification of delamination, emissivity properties appear to contribute to stabilizing phase signals. These findings underscore the importance of accounting for paint properties in conservation diagnostics.