Dry film thickness, or DFT for short, is the most concrete measure indicating whether a coating system has been applied in accordance with the specification. Visual inspection alone is not sufficient; the system will not be accepted without thickness measurement.
Why it is measured
The barrier performance of the coating is largely dependent on its thickness. Areas below the target will corrode earlier than expected. Thickness significantly above the target can lead to curing problems, cracking, and loss of adhesion. Therefore, DFT is evaluated in terms of both lower and upper limits.
Difference between ISO 2808 and ISO 19840
ISO 2808 defines the methods by which film thickness is to be measured in paints and varnishes. ISO 19840 addresses how measurements are to be made and results evaluated on steel structures, i.e., on rough blasted surfaces.
The difference is practically significant: on a blasted surface, the measuring device tends to take readings from the peaks of the profile. Therefore, a correction value dependent on the surface profile is applied, and the measurement result is evaluated accordingly.
Measurement frequency and acceptance
The number of points to be measured and how the acceptance criteria will be applied are determined by the contract. If the client's specification requests a method such as SSPC-PA 2, the measurement frequency is planned accordingly. It is essential that the measurement points are evenly distributed across the surface, and readings are not taken only from easily accessible areas.
Critical details are evaluated separately. The coating naturally thins at edges, welds, bolts, and around holes; stripe coats are applied to these areas according to the specification and checked.
Measurement is a record
DFT measurement is not just a number but part of the handover documentation. Wet film thickness is recorded during application, and dry film thickness is recorded on a per-coat basis. These records are linked to the work order and collected in the final acceptance file; thus, the application becomes traceable afterward.
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