## 2 Materials and Methods
### Testing and Characterization  
The coating impact resistance was determined using a QCG-50 coating impact tester (Ningbo Yinzhou Rijin testing equipment Co., Ltd., Ningbo, China). The coating of the sample was positioned horizontally on the iron plate, and the weight hammer was fastened on a certain height of the sliding cylinder through the control device. As the control button was pressed, the heavy hammer would fall freely on the press. When lifting the heavy hammer to take out the experimentation plate, the deformation degree of the coating surface was observed, and the height of the heavy hammer falling on the experimentation plate was recorded. Impact resistance referred to the maximum height on the test plate without damaging the coating. The adhesion was tested according to ISO 2409-2007 [20]. The coating adhesion was determined with a QFH-HG600 coating scriber (Zhongxi Yuanda Technology Co., Ltd., Beijing, China). The handle was held and a multiblade tool was used on the plate. The coating was cut at 20-50 mm/s under uniform pressure. Then, the manipulate was repeated by rotating it 90\({}^{\circ}\) to form a 1 mm \(\times\) 1 mm grid pattern. The 3M brand 600-1 PK transparent pressure-sensitive adhesive tape (19 mm wide) was applied to the entire grid and revealed at a minimum angle. The coating damage was observed. The adhesion level was judged in line with the damage degree. The adhesion of the coating was 0, which made known that the coating adhesion was the best. Coating hardness was determined via a QHQ-A coating hardness tester (Flora Automation Technology Co., Ltd., Tianjin, China). Coating gloss was determined with a BGD512-60 gloss meter (3 NH Technology Co., Ltd., Shenzhen, China). The color of the coating shall be tested using a SEGT-J colorimeter (Shenyang Guoti Precision Testing Instrument, Liaoning, China). L, a* and b*, respectively, stand for the lightness, red-green axis color index and yellow-blue axis color index of the Al plate, while L, a* and b* represent the surface coating of Al plate. \(\Delta\)L, \(\Delta\)a* and \(\Delta\)b* represent the lightness difference, red-green index difference and yellow-blue index difference of the two samples, respectively. According to the color difference Formula (1), the color difference \(\Delta\)E was computed:  
\[\Delta\text{E}=[(\Delta\text{L})^{2}+(\Delta\text{a*})^{2}+(\Delta\text{b*})^ {2}]^{1/2} \tag{1}\]  
Coating microstructure was characterized with SEM. A Vertex 80V infrared spectrometer was used to measure the influence of different Al powder concentrations on the waterborne UV-curable coating. Far-infrared emissivity of the coating in the range of 8-14 \(\upmu\)m was determined using an IR-2 far-infrared emissivity tester (Chinese Academy of Sciences Shanghai Institute of Technological Physics, Shanghai, China). The emissivity measurement instrument uses an active blackbody radiation source to measure the normal reflectivity of the object to be measured, and then measures its normal emissivity in the 8-14 \(\upmu\)m infrared band. First, the temperature was set to 250 \({}^{\circ}\)C, the power was 43% and the calibration started after the value was stable. Then the copper reference plate was placed so that the emissivity was 0.45. The aluminum-coated compensation reference plate was placed on the bracket directly below the test head, and the mirror reflection (MR) was adjusted to make the emissivity reach 0.05. After adjustment, the infrared emissivity of the coating was tested. The corrosion resistance of the waterborne UV-curable coating was determined by a CHI660E electrochemical workstation (Beijing Jinyang Wanda Technology Co., Ltd., Beijing, China) in line with GB/T 15748-2013 [21], and the NaCl concentration was 3.5% at room temperature. A three-electrode cell structure was utilized for the electrochemical experiment. A saturated calomel electrode (SCE) was used as the reference electrode and a platinum electrode as the counter electrode.