*Article* **Evaluation of Coating Film Formation Process Using the Fluorescence Method**

**Ayako Yano \*, Kyoichi Hamada and Kenji Amagai**

Department of Mechanical Science and Technology, Gunma University, Gunma 376-8515, Japan; t171b070@gunma-u.ac.jp (K.H.); amagai@gunma-u.ac.jp (K.A.)

**\*** Correspondence: yano@gunma-u.ac.jp

**Abstract:** In this paper, we invented a novel observation method of the coating film formation process using the fluorescence method. With this method, the temporal change in the coating film thickness can be evaluated quantitatively. In addition, since the thickness and flow of the coating film can be measured simultaneously, the detailed coating film formation process was clarified. In the experiment, the adhesion behavior of the spray-paint droplets when applied to a wall was investigated. The characteristics of coating films formed by the spray droplets, particularly the influence of injection pressure on the coating film formation, were determined using the fluorescence method. At the initial stage of the coating process, the coating area increased linearly. When the ratio of the coating area to the measurement range reached about 80%, the rate at which the coating area increased slowed down, and an overlap began. The amount of paint that adhered to the coating film formation could be estimated by calculating the overlap ratio. Moreover, the thickness and smoothness of the coating film were evaluated using the histogram data of the fluorescence intensity. The leveling process was discussed in relation to the standard deviation of the histogram data. In addition, the flow of the paint during the coating film formation was investigated using tracer particles, and the effect of the spray gun injection pressure on the leveling process was investigated. Changes in the film thickness and flow during the coating film formation process could be evaluated through fluorescence observation.

**Keywords:** spray coating; coating film formation; leveling of coating surface; fluorescence method; visualization
