Dry film thickness measurements on abrasive blast-cleaned steel are influenced by the roughness of the substrate. ISO 19840 provides a structured approach for verifying dry film thickness against nominal dry film thickness on rough surfaces. Where the ISO 8503-1 surface-profile classification is being used, the associated correction values are: Fine: 10 µm Medium: 25 µm Coarse: 40 µm These values are corrections applied to account for the influence of the roughened substrate on the instrument reading. They are not measurements of the actual surface-profile height. The correction is applied when deriving the individual dry film thickness from the instrument reading. The standard is applicable where the nominal dry film thickness is 40 µm or greater. Inspectors should establish the applicable measurement and correction approach before interpreting DFT readings and should also consider the project specification and coating-system requirements.