Accelerated lifetime testing of materials used in solar thermal systems is highly important for providing quality products to the end user, thus creating trust in the technology. Currently, there is no standard procedure to assess the durability of selective solar absorber coatings used for unglazed collectors. Existing tests (ISO 22975-3) are designed for glazed solar collectors where the micro-climate differs considerably. Temperature frequency distributions (TFD) and time of wetness (TOW) frequency distributions for the absorber surface, which are the basis of accelerated aging tests, were generated with TRNSYS simulations and extracted from measurement data of two large solar installations in Geneva (CH). Further, three types of coatings with potential for being used in unglazed collectors were tested in high humidity and condensation conditions.