The ASTM D3258-04 (2019) standard test method is suitable for determining the porosity of white or near-white paints films by staining. This method evaluates the degree of dyestuff penetration by treating the film with a dyestuff solution, thereby indirectly reflecting the porosity distribution and compactness of the film. The standard specifies the test materials, equipment, dyestuff preparation, operation procedures, and results evaluation methods in detail to ensure the test Repeatability and accuracy. This method can be used for quality control, Coating performance optimization, and comparative analysis to provide a critical reference for the performance of films in protective, decorative, and functional applications.