To clarify the deterioration in coating materials exposed to outdoor conditions and the preventive effect of coating materials against carbonation and corrosion of reinforcing bars in reinforced concrete members, we conducted a 20-year outdoor exposure test using a model building constructed from concrete specimens with various finishing materials. We used W/C 55% and 70% concrete specimens and applied various coating materials and penetrants to the surfaces. According to the results of the evaluation of the deterioration of the coating materials, there was some peeling and cracking in thin coating materials; however, no remarkable deterioration of thick coating materials was observed. Some cracks were observed in the surface layer of some multi-layered coating materials, but the main coating layers were almost sound. We also confirmed that fluorine coating materials have a high weatherability. A correlation between the degree of chalking and the degree of peeling and cracking in the coating materials was found. According to previous studies, the limit state of an organic coating material is expressed by the extent of gloss retention. In these test results, we observed a high correlation between the gloss retention and the extent of deterioration of the coating material. It seemed that gloss retention was a useful indicator of deterioration of the coating material. We also investigated the preventive effect of the coating material against carbonation in concrete. We confirmed the effect of using coating materials based on the outdoor exposure results. The carbonation preventive effect as evaluated by an accelerated carbonation test was found to be different from the effect as evaluated by the outdoor exposure test because of the penetration of rain water into the applied concrete. Evaluation of the carbonation preventive effect of a coating material should take into account the influence of the moisture content in the applied concrete. Deterioration of the coating material was affected by the carbonation preventive effect except in the case of high-permeability coating materials. We satisfactorily estimated the depth of carbonation in concrete with a deteriorated coating material using the equation proposed by a previous study. Finally, we estimated the corrosion of reinforcing bars using the corrosion indicator method. We evaluated the observed corrosion depth of iron particles previously mixed in the concrete and took this as an indicator of the reinforcing bar corrosion. From the results, it was observed that some observed depths after 20 years were shallower than the depth after 11 years. It seemed that rain water had the effect of diffusing calcium hydroxide from the concrete core to the surface. All specimens were observed to have corroded iron particles in the carbonated area of the concrete. It seemed that the corrosion preventive effect of a coating material was mainly to delay the progress of corrosion of the reinforcing bar.