GB/T 1731-2020 "Test method for flexibility of paint films and putty films" is applicable to the determination of flexibility of paint films and putty films. It employs a cylindrical mandrel bending tester, where painted test panels are bent around cylindrical mandrels of different diameters to observe whether cracking or peeling occurs in the coating. The standard specifies a mandrel diameter range of 0.5 mm to 50 mm, with the test conducted in an environment of 23 ± 2°C. Observations are completed within 10 seconds after the test panel is bent. The standard clarifies the requirements for test panel preparation, the steps for bending operations, and the criteria for result evaluation. Different mandrel diameters correspond to different flexibility grades, providing a basis for assessing the deformation resistance of coatings.