Wet film thickness gauges (WFT)

Wet film thickness gauges (WFT — Wet Film Thickness) are simple but indispensable mechanical instruments for checking the thickness of paint and coatings immediately after application, while the material is still liquid. Checking the wet thickness is fundamental to estimating the final dry thickness in real time, through the volume solids ratio specified by the paint manufacturer, and it allows the application process to be corrected at once (adjusting the spray, brushing or rolling) before drying.

Notched combs

The most widespread instrument is the notched comb (also called a wet film comb): a steel or plastic blade with calibrated notches of graduated height (for example 25, 50, 75, 100, 125, 150, 200 μm and upwards). The comb is placed perpendicular to the freshly painted surface; the highest notch that leaves a mark in the wet film identifies a thickness interval (the real thickness lies between the highest notch that does not touch and the lowest notch that does). Typical resolution is 25 μm for standard combs and 10-15 μm for high-resolution ones. Combs are available in metric and in mils (thousandths of an inch).

Wheel gauges

There are also eccentric wheel instruments (wet film wheel gauges) which, thanks to a cam geometry with calibrated eccentricity, measure the thickness at finer resolution and continuously. They are used in high-quality industrial painting applications where the resolution of the standard comb is not sufficient. Most WFT instruments are of stainless steel or aluminum, with a laser-marked graduated surface for durability and legibility. Folding and pocket models are available for portability.

Calculating the dry thickness

The wet thickness value (WFT) is converted into the expected dry thickness (DFT) through the paint’s volume solids ratio (Volume Solids %), declared by the manufacturer in the technical data sheet: DFT = WFT × (Volume Solids / 100). For example, a paint with 50% volume solids applied at 200 μm WFT will give roughly 100 μm DFT once the solvent has evaporated. This predictive check allows immediate corrections to the rate of application and avoids the final coating being under- or over-thickness.

Applications

WFT gauges are used in every industrial painting application where real-time control during application is required: corrosion protection of steel structures, painting of structural steelwork, sandblasting + primer + topcoat in marine and offshore systems, application of industrial floor coatings (epoxy, polyurethane), anti-slip treatments and intumescent fire-protection paints. They are also indispensable in the training courses for FROSIO- and NACE-certified industrial painters.

Standards and selection criteria

WFT measurement is governed by standards ISO 2808-1 (method 1A comb, 1B wheel) and ASTM D4414. Calibration of combs is carried out with certified shims or by dimensional inspection of the notches on a CMM. The choice of instrument depends on the range of thicknesses applied (so as to cover the notches typical of the coating system), on the resolution required, on robustness (steel for intensive use, disposable plastic for economy) and on chemical resistance to the type of paint used.