In automotive engineering, paint evaporation is considered one of the major sources of volatile organic compound emission. The rotary bell atomizer (RBA) is a special atomizer used for improved automotive painting. A model to predict the droplet size was developed by combining two already-reported models. Further, the calculated values of evaporation rate constant, K, and the RBA nozzle parameter, K 1 , for water and clearcoat at room temperature (24 °C) are presented here. K 1 was dependent on the flow rate and bell speed, but its variance for clearcoat was less than the measurement uncertainty. The Sauter mean diameter of the particle leaving the RBA at different flow rates and bell speeds can be predicted using K 1 . The values of K were 1.3 × 10 −5 to 2.0 × 10 −5 cm 2 /s for clearcoat and 8.6 × 10 −6 to 1.53 × 10 −5 cm 2 /s for water. A generic model which can anticipate the evaporation rate of droplets with time length was also introduced.