The ASTM D3258-04 (2019) standard test method is used to determine the porosity of white or near-white paint films. This method measures the porosity of coatings by staining reagents to evaluate their resistance to penetration. The testing steps include coating the paint film, measurement reflectance after drying, application of staining media, removal of excess media, and measurement reflectance again. The difference between the two Measured values reflects the porosity of the coating. The lower the porosity, the stronger the anti-penetration performance. This method helps evaluate the actual performance of Coatings, especially in application scenarios where anti-penetration is required.