**4. Conclusions**

In this study, we investigated the process of forming a coating film by changing the paint injection pressure. In addition, the thickness and smoothness of the coating film were investigated using the fluorescence method. The coating could be realized using the fluorescence method by mixing a fluorescent agent with the paint. The higher the injection pressure, the shorter the time until the coating was formed. The area ratio rises, linearly, immediately after the start of droplet deposition, but reached 100%, while decreasing the increase rate of the area ratio by overlapping. It was possible to estimate the amount of paint that overlapped and adhered using the overlapping ratio. The thickness of the coating could be calculated from the fluorescence intensity. Additionally, the smoothness of the coating film could be evaluated by examining the standard deviation of the fluorescence intensity. The thickness of the coating increased, linearly, from the start of droplet deposition to the end of spraying. The standard deviation increased near the onset of droplet deposition and then decreased until the time of film formation. At the end of the injection, the standard deviation decreased, and a very smooth coating was formed. The flow of the coating film formation was observed by tracking the fluorescent particles in the paint. We found that the higher the injection pressure, the more active the flow of the coating film. In this study, the thickness of the coating film and the flow of the formation process could be observed in detail at the same time using fluorescence observation. By using the methodology of this paper, it is possible to quantitatively analyze the coating film formation process. This method is applicable for various industrial processes, such as the painting of automobiles, ships and buildings. The conditions for forming a smooth and high-quality coating surface with a smaller amount of paint can be determined on the basis of data rather than empirical rules.

**Author Contributions:** Conceptualization, K.A.; investigation, A.Y., K.H. and K.A.; writing—original draft preparation, K.H.; writing—review and editing, A.Y. and K.A.; supervision, K.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.
