The application of easy-to-clean coatings grows in polluted urban and industrial areas and paint suppliers and coil coaters develop coil coated products with improved easy-to-clean properties. However, there is not any standardized procedure allowing for prediction of the degradation of the easy-to-clean properties in service conditions yet. Dirt retention is usually tested in laboratory conditions using a carbon slurry test, which exists in many modifications. The aim of this study was to compare results of several carbon slurry tests to outdoor exposures at four sites representing different climates and pollution characteristics for a large number of easy-to-clean and reference materials in order to propose the optimal testing procedure. Six procedures of the carbon slurry test were used differing in the type of carbon black, drying time after the slurry application, baking temperature and baking time. The results were correlated to dirt pick up and dirt retention measured after 3, 6, 12 and 24 months of exposure at a non-polluted marine site, non-polluted marine tropical site and two sites with high industrial pollution and dirt deposition. A procedure of the carbon slurry test providing the best prediction of outdoor performance was indentified. It is characterized by a prolonged baking time. Still, the acquired data suggest that a short-term 3-months exposure at a site with high industrial pollution was more reliable for prediction of dirt pick up and dirt retention after 2 years of exposure than the best performing carbon slurry test.