One of the major problems of current accelerated weathering methods is difficulty of reproducing the results of in-use exposure. This paper reports a microanalytic method used to improve that reproducibility. For that purpose, electron-microscopic observations were performed of coating surface degradations caused by various accelerated weathering methods and those induced by outdoor exposures, along with those of in-use car panels, thereby identifying differences in film degradation forms.The accelerated weathering tests demonstrated the effects that the wavelength distribution of radiation, temperature, water feeding procedure, etc. had on loss of gloss, fading, and film surface degradation forms, especially a great effect of ultraviolet light on such failures. It was based on analytic results of such environmental factors that the accelerated testing conditions including wavelength distribution of the radiation, water feeding, temperature, etc. were improved, thus making possible better evaluation and prediction of gloss retention life of solid coatings.