The ISO 19840 standard specifies how dry-film thickness on rough surfaces is measured and accepted when a protective coatings system is used for corrosion protection of steel structures. The standard describes in detail the Measuring method and instrument requirements, emphasizes the effect of Coating thickness on anti-corrosion effect, and provides acceptance criteria for Coating thickness on different asperity surfaces. Through these specifications, it is ensured that the coating is evenly covered on the steel structure to provide effective long-term corrosion protection and meet quality control and service life requirements. The standard is widely used in the painting of steel structures such as buildings, bridges, ships, etc. It is an important technical basis for ensuring anti-corrosion performance.