Photo-degradation of a polyurethane-based topcoat induced by accelerated Xe-lamp irradiation is studied using Doppler broadened energy spectra (DBES) and positron annihilation lifetime (PAL) spectroscopy coupled with slow positron technique. Significant and similar variations of S-parameter and ortho-positronium intensity (I-3) on the coating are observed as functions of depth and of exposure time. Cross-link densities have been measured by the solvent-swelling method. A correlation between the increase of crosslink density and a reduction of free-volume and hole fraction during the degradation is observed.