DFT meaning

Dry film thickness (DFT) is the thickness of a coating as measured above the substrate. This can consist of a single layer or multiple layers.

DFT gauge / meter

DFT meter or gauge is used to measure the coating thickness on Ferrous or Non-Ferrous metal substrates using electromagnetic induction and/or eddy current principles.

How do you measure coating thickness?

Dry Film Thickness can be measured on either magnetic steel surfaces or non-magnetic metal surfaces such as stainless steel or aluminium using a digital coating thickness gauge. The principle of electromagnetic induction is used for non-magnetic coatings on magnetic substrates such as steel. The eddy current principle is used for non-conductive coatings on non-ferrous metals substrates.

What is coating thickness?

Coating Thickness can be split into wet film thickness and dry film thickness. The coating thickness is vital in the quality control process. Usually specified by the paint manufacturer, asset owner or third party, who will specify the paint and the specifications.  Ways to measure coating thickness ranges from wet film combs for wet film to sophisticated digital coating thickness gauges for dry film.

What is dry film thickness?

Dry film thickness (DFT) is the thickness of a coating as measured above the substrate. This can consist of a single layer or multiple layers. DFT is measured for cured coatings (after the coating dries). The thickness of a coating depends on the application and type of process employed.

What is wet film thickness?

Dry film thickness, or DFT is a measurement of the dry and cured material after all liquid has evaporated. Wet film thickness, or WFT is the measured thickness of any applied wet paint that is liquid-based.

How do you measure wet film thickness?

WFT is measured by using Combs or Gauges by the applicator as the coating is being applied to ensure that the measurement is representative of the calculated wet film before significant solvent evaporation occurs.

Does a textured profile affect the accuracy of a destructive DFT (Dry Film Thickness) tester?

There will always be a level of “uncertainty” with this method. The choice of blade, position of the cut, point along the cut where measurement is taken and the factor used to convert 1 reticule to x micrometres have an impact of the final reading. Technically at least 3x readings should be taken along a single cut and the average of DFT for each layer then taken as the DFT for that area.

The surface profile will have an impact on the final reading. On a steel surface this is considered reasonably negligible but on concrete the range from peak to valley can be significant – Hence the averaging of several readings along a cut length.

Does a pure acrylic coating system experience a reduction in film thickness over time due to degradation or other factors? If a DFT measurement is taken several years after the initial application, will the reading be lower than the original thickness?

All coating systems degrade with age to some extent. While acrylic systems are generally more resistant to UV and chemical exposure, the different chemistry of different acrylic systems presents differing ageing characteristics. Acrylics, like epoxy-based systems, will undergo a degree of chalking which will lower the DFT over a prolonged time. Of more concern, again referring to the right-hand picture, is what appears to be cracking of the film. Judging the degree of degradation is not a simple task but consideration of environmental factors plays a significant part.

When using a destructive DFT tool, is it possible to manipulate the results to find a specific thickness? For example, could someone intentionally measure a thicker or thinner coating by adjusting the testing location or technique?

No, I would not think so if they were following the instructions. It would be important to make sure you use the correct blade and the resolution is correct and that you are using a blade closest to the total range. You would quite easily be able to determine whether a contractor has put on one coat instead of 2 coats for example.

Destructive DFT is a recognised inspection technique and, while it is acknowledged that it is not as accurate as other techniques, it is still widely used where ither techniques cannot be used.

Surface Profile

Surface profile is a measurement of the peak-to-valley height, by using a surface profile gauge. Surface roughness, on the other hand, is the combined measurements of the surface profile and the frequency of the peaks across a linear length (also known as the peak count).

Surface Roughness

Surface roughness, often shortened to roughness, is a component of surface texture. It is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth. Surface roughness is measured using a stylus instrument.