ISO 19840:2004 specifies a procedure for the verification of dry film thickness against the nominal dry film thickness on rough surfaces, including the adjustment of the instruments used, the definition of inspection areas, sampling plans, measurement methods and acceptance/rejection criteria. The procedure is based on the use of instruments of the permanent magnet and inductive magnet type, adjusted to zero and to a known thickness on a smooth surface. It does not apply to nominal dry film thicknesses less than 40 micrometres.