## 2 Materials and Methods
### Testing and Characterization  
The MA-5 QC chromatic meter (X-Rite Pantone Co., Ltd., Shanghai, China) was used to test the color difference of water-based topcoat paint film. The L\({}_{1}\), a\({}_{1}\) and b\({}_{1}\) of one point on the surface of the paint film were measured by the chromatic meter, and then L\({}_{2}\), a\({}_{2}\) and b\({}_{2}\) of another point on the surface of the paint film were measured, too. The color difference is calculated according to the formula: \(\Delta\mathrm{E}=[(\Delta\mathrm{L})^{2}+(\Delta\mathrm{a})^{2}+(\Delta\mathrm{ b})^{2}]^{1/2}\). The AIBEVS 1502K trigonometric glossometer (Guangzhou Shenghua Industrial Co., Ltd., Guangzhou, China) was used to test the gloss of water-based topcoat paint film. The glossiness meter was placed on the surface of the paint film, and the glossiness of the paint film can be measured. The pencil hardness tester (Shenzhen Jiangcheng Instrument Co., Ltd., Shenzhen, China) was used to test the hardness of water-based topcoat paint film. The pencil was placed on the mechanical trolley and fixed with a clip to keep the instrument level with the paint film. The tip of the pencil should contact the surface of the paint film, and then the trolley was pulled at a uniform speed to move the tip of the pencil across the paint film. If there is a scratch of more than 3 mm, the hardness of the paint film is the corresponding pencil hardness. If there is no scratch, a higher hardness of the pencil was used to measure, until the scratch is more than 3 mm. The film scriber (Shenzhen Xinhengsen Instrument Equipment Co., Ltd., Shenzhen, China) was used to test the adhesion of water-based topcoat paint film. First, the tool was perpendicular to the sample surface and the sample surface was cut at a uniform speed, then rotated 90\({}^{\circ}\) and cut in the same way once, so that the sample surface was cut into a grid pattern. The tape was adhered to the grid pattern and torn off at an angle of 60\({}^{\circ}\) within 0.5-1.0 s, and the peeling of the paint film on the tape was observed. The impact testing machine (Jinan Ruima Testing Machine Manufacturing Co., Ltd., Jinan, China) was used to test the impact resistance of water-based topcoat paint film. The test board was placed on the impact testing machine base, the small iron ball was fixed to a height, and the control button was pressed so the small iron ball would freely fall to the test plate. The test board was taken out and it was observed whether there were cracks and spalling on the test board after being hit by the small iron ball. The mechanical testing machine (Dongguan Bolaide Instrument Equipment Co., Ltd., Dongguan, China) was used to test the tensile strength at break of water-based topcoat paint film. The paint film was placed in the fixture and clamped to ensure that the paint film will not slide, and then, under a certain longitudinal load at the tensile speed of 0.12 mm/min, it deforms and breaks. The tensile strength at break = (elongation of paint film/original length of paint film between clips) * 100%. The 15% NaCl, 70% medical ethanol (Shandong Duofeng Chemical Co. Ltd., Linyi, China), detergent (LIbY Group Co., Ltd., Hangzhou, China) and red ink (Weifang Meicai Inkjet Technology Co., Ltd., Weifang, China) were used to test the liquid resistance of water-based topcoat paint film. First, the filter paper was placed in the solution for 30 s, and then the filter paper was placed on the surface of the paint film and covered with a glass cover. After one day, the filter paper was removed and the residual water on the surface of the paint film was blotted with absorbent paper, and it was checked whether there were traces of discoloration and other damage in the tested area.