**1. Introduction**

Reducing the emission of volatile organic compounds (VOCs) generated in various industrial fields is crucial, due to its adverse effects on humans and the atmosphere. The World Health Organization (WHO) has defined organic compounds with a boiling point between 50 and 260 ◦C, such as toluene, xylene, and formaldehyde, as VOCs. Air pollution caused by suspended particulate matter (SPM) and photochemical oxidants is the problem caused by VOCs, which is still a serious situation. In addition, VOCs are involved in the generation of fine particles (PM 2.5) as a precursor. VOCs are primarily released during painting processes, because organic solvents, such as thinner, are used to dilute the paint. Thinner contains xylene, toluene, etc., which can easily be released into the atmosphere. In particular, spray coating is performed as a coating method for large machines, such as automobiles; however, paint consumption is enormous, and a large amount of VOCs are generated. In addition, VOC emissions from shipbuilding, building construction, and automotive production have been reported [1–3], and the effect on humans has been discussed [4,5].

To reduce VOC emission during the painting process, it is important to improve the adhesion efficiency of the paint and optimize the coating time. Therefore, clarifying the relationship between the characteristics of the paint spray and the paint surface quality is necessary. Coating process optimization technology could reduce paint consumption

**Citation:** Yano, A.; Hamada, K.; Amagai, K. Evaluation of Coating Film Formation Process Using the Fluorescence Method. *Coatings* **2021**, *11*, 1076. https://doi.org/10.3390/ coatings11091076

Academic Editor: Cecilia Bartuli

Received: 10 August 2021 Accepted: 30 August 2021 Published: 6 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

while maintaining surface quality [6–8]. Additionally, spray-coating modeling has been investigated as a means of predicting coating thickness [9]. Research has been conducted to understand the mechanism behind coating film formation using spray coating [10,11]. However, in conventional studies, observations have been made with the naked eye and with a camera, and only the area and shape of the paint covering the observation range have been measured. The coating thickness and paint flow during the coating film formation process have not been reported in previous studies. In this study, we investigated the coating process in which the atomized paint adheres to the coated surface. Further, we propose a methodology to determine the change in thickness and smoothness of the coating film quantitatively using the fluorescence method. Fluorescence has been used in various fields, such as biology, medicine, and combustion diagnostics in engineering [12]. However, fluorescence has not been previously applied as an evaluation method to elucidate the paint film formation mechanisms. In this study, the fluorescence method is newly applied to the coating technology.
