## 5 Conclusions  
In this paper, the epoxy resin coating on a 304 stainless steel surface was cleaned using water jet-guided quasi-continuous laser cleaning technology. First, the cleaning mechanism of the water jet-guided quasi-continuous laser was analyzed, and then the effects of energy density, spot overlap rate, and laser pulse frequency of the water jet-guided laser on the cleaning effect were studied. The conclusions are as follows:  
1. The energy density and spot overlap rate of the water jet-guided laser have an obvious influence on the surface integrity of the substrate after cleaning. When the spot overlap rate is 70%, the complete cleaning threshold of the resin-based coating is between 12.5 J/cm\({}^{2}\) and 17.5 J/cm\({}^{2}\) of the energy density of the water jet-guided laser, and reaches the damage threshold of the metal substrate when the energy density is 20 J/cm\({}^{2}\). With an increase in the spot overlap rate, the thermal accumulation effect caused by laser irradiation will reduce the thermal damage threshold of the metal substrate.
2. After the coating is removed by the action of the water jet-guided laser, the surface of the substrate appears as a large number of craters with a depressed center and raised surroundings. The formation of craters is mainly the result of the combined effects of surface melting and shock waves. As the laser energy absorbed by the metal substrate increases, the crater size gradually increases, and the oxygen content and surface roughness of the substrate surface also increase.
3. The relationship between the peeling amount of epoxy resin coating and process parameters (energy density of water jet-guided laser, spot overlap rate, and laser pulse frequency) shows that the removal rate of the coating increases with the increase of energy density and spot overlap rate of water jet-guided laser and the decrease of laser pulse frequency. When the energy density of the water jet-guided laser is 17.5 J/cm\({}^{2}\), the transverse cleaning speed is 135 mm/s, the longitudinal cleaning line spacing is 0.1 mm, and the laser pulse frequency is 900 Hz, the coating is better removed.
4. The mechanism of water jet-guided quasi-continuous laser cleaning of epoxy resin coating on metal substrate is a multi-effect coupling mechanism, mainly including the thermal ablation effect, laser plasma shock effect, thermal stress vibration effect, and water jet erosion effect. These mechanisms interact with and influence cleaning quality in the cleaning process. Under conditions of low energy density and low spot overlap rate, the cleaning mechanism is mainly a thermal ablation effect. Under the conditions of high energy density and high spot overlap rate, the cleaning mechanism is mainly the combined effect of the thermal ablation effect and the plasma shock effect.  
Conceptualization, Q.L. and Y.Z.; methodology, Q.L. and Y.Z.; software, K.W. and C.C.; validation, J.M., G.Z., L.L. and G.L.; data curation, K.W., Z.Z., C.C. and D.D.; writing--original draft preparation, Q.L.; writing--review and editing, L.L.; supervision, Y.Z.; project administration, Y.Z.; funding acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China (Grant No. 51875328) and the Natural Science Foundation of Shandong Province (Grant No. ZR2019 MEE013). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data availability Statement: Not applicable. Acknowledgments: The authors thank the National Natural Science Foundation of China and the Natural Science Foundation of Shandong Province for help identifying collaborators for this work.