Dry Film Thickness (DFT) is the coating thickness measured above the substrate. This can consist of a single layer or multiple layers. Measure the DFT of the cured coating (after the coating has dried). The thickness of the coating depends on the application and the type of process employed. Proper thickness should be determined by recommended coating system parameters.
Coatings are a key component of an anti-corrosion coating system. The main function of the coating is to provide corrosion protection by adhering to the substrate. It cannot be overemphasized that the thickness applied is sufficient to ensure corrosion protection as well as proper adhesion.
Measure film thickness to ensure compliance with ASTM, ISO and other specifications. Coating inspection tools are used at every stage of the coating process to ensure that DFT meets the correct specifications.
The manufacturer's recommended maximum coating thickness is usually based on the coating's sag resistance. DFT is a parameter considered during quality control or inspection. The durability of the coating is a function of the DFT, while the thickness of the individual coatings in the coating system and the reported DFT of the entire system specify the durability of the coating.
Types of DFT gauges include:
Magnetic pull-off (Type I)
Constant pressure probe (Type II)
Electron flux
Vortex meter
Standards for DFT measurements include:
ISO 2808
BS 3900 Part C5
BS 5494
ASTM D1005
D1400
D2691
SIS 184160
Even basic protective coating specifications require measuring DFT. It is considered an important factor in determining the durability of coating systems. Each coating thickness in the system and the DFT of the overall system need to be measured and recorded to show that the specified system has achieved the required durability. Therefore, both the contractor and the inspector need to know how many measurements to take during the inspection. Other aspects of DFT measurements performed against the standard include calibration of gauges and the effect of surface profile on thickness measurements. The number of areas to measure coating thickness depends on the size of the coated area.
There are two tools used when measuring coating thickness - wet film thickness (WFT) gauges and dry film thickness (DFT) gauges . The WFT gauge is used to test the thickness of liquid-based wet coatings and should be used at the time of coating application to ensure that the wet film is properly measured before the solvent evaporates significantly. To ensure accurate readings, use the DFT gauge only when the film reaches fingerprint hardness.
X-ray fluorescence, coulometric, beta backscattering, magnetic induction, amplitude eddy current and phase sensitive eddy current are popular and complex plating and other coating thickness applications