Wet Film Thickness or WFT is the measured thickness of any applied liquid-based wet paint. When coating, the applicator should use a wet Film Thickness Gauge to ensure that this measurement represents the calculated wet film before significant solvent evaporation occurs. Even a slight delay in taking wet film thickness measurements can result in falsely low readings as the solvent may have evaporated from the film before the measurement is taken, which is why WFT gauges are largely considered application tools rather than inspection instrument reason.
Why is WFT important? By measuring the WFT of the paint, the applicator can adjust the gun speed, number of coats, and make gun adjustments (if possible), or select a different spray head to apply the correct amount of paint to achieve the specified dry film thickness.
What is the relationship between WFT (wet film thickness) and production? time is money. Build-up coatings may need to be applied, or worse (by sanding) reduced in thickness, which hinders production and reduces profitability. While insufficient thickness can usually be corrected by adding more coats, excessive thickness can lead to solvent entrapment, sagging and sagging which, if not corrected, can lead to bonding problems. Each of these events can negatively impact project schedules. Perhaps even more important to remember: The performance of most coatings is based on achieving a specified dry film thickness, and applying the correct wet film thickness can help meet this requirement. Measuring wet film thickness during application can immediately determine that applicators need to make in-process adjustments.
How is WFT calculated? Paint manufacturers may specify on the Product Data Sheet (PDS) the range of wet film thicknesses to obtain the desired dry film. However, many manufacturers only list a suggested DFT because the amount of diluent that the contractor will add is unknown and that amount affects the target WFT. Specifications usually list the desired end result (DFT), not the method/approach (WFT) to achieve that goal. A wet film thickness target (or range) can be calculated. Equations with and without the addition of finer WFT are shown. The dry film thickness range (per coat) is extracted from the specification or the coating manufacturer's PDS (whichever is the control document), and the volume solids are listed on the PDS.
What are the volume solids in paint? Expression of the film-forming component of the S content of the coating in volume solids, or the left side of the material behind after the solvent has evaporated from the applied coating. On a very basic level, paint consists of solvents, resins, pigments and additives. Volume solids is the percentage of formulation that is non-volatile and will remain on the surface after the coating has dried and cured.
Without thinner:
Wet film thickness (WFT) = dry film thickness (DFT) ÷ volume percent solids
example:
Stated DFT = 3-5 mils
Volume Solids = 65% (0.65)
WFT = 3÷0.65 = 4.6 mils; 5÷0.65 = 7.7 mils
Based on this example, assuming the applicator applies a WFT of 5 to 8 mils, a specified DFT of 3-5 mils should be achieved.
The equations used to calculate target WFT with thinner added in the shop or field require that the volume solids content of the coating (as manufactured) need to be adjusted for the volume of thinner added (as a percentage of the total coating volume).
With thinner:
WFT = DFT ÷ (volume solids content ÷ 100% + percent of diluent added)
example:
Stated DFT = 3-5 mils
Volume Solids = 65%
Thinner by volume = 20% (for example, add 2 gallons of thinner to 10 gallons of mixed paint)
Step 1: (0.65÷1.20) = 0.54, or 54% adjusted volume solids based on 20% diluent addition
Step 2: 3 ÷ (0.54) = 5.6 mils 5 ÷ (0.54) = 9.3 mils
Based on this example, assuming the applicator has a WFT between 6 and 10 mils, even if you dilute it by 20%, the DFT should still achieve the specified DFT of 3-5 mils, which is part of the wet film rather than the dry film.
How is WFT measured? Wet Film Thickness Gauges are used to monitor the thickness of wet coatings applied to achieve a specified dry film thickness. They measure all types of wet organic coatings such as paints, varnishes and varnishes on flat or curved, smooth surfaces. These gauges are usually measured in microns (mil) or mils. Wet film thickness was measured according to ASTM D4414, Standard Practice for Wet Film Thickness Measurements by Notch Gauge. Pictured is a gage typically used to measure WFT.
How are WFT gauges used? Using the WFT gauge is very simple. First, verify that the notches (teeth) are clean and free of any dried paint. Immediately insert one end of the gauge vertically into the wet coating. The teeth at both ends will penetrate down to contact the underlying surface and be wetted by the coating. Take out the gauge and read the highest wetness step. If none of the numbered notches contain wet coating, rotate the head of the gage to a lower WFT range and re-measure. If all numbered notches contain wet coating, rotate the head of the gage to a larger WFT range and re-measure. A schematic depicting the proper use of the WFT gauge is shown. After each reading, the teeth of the gauge need to be wiped.
What units are used when measuring WFT? Typical units of measurement are mils and microns. A mil is a unit of length equal to one thousandth (10 -3) of an inch (0.0254 mm). A micron is a unit of measurement for a length equal to 0.001 mm (approximately 0.000039 inches). Its symbol is µm. 25.4 microns equals 1 mil.
Conclusion: Calculating and properly measuring wet film thickness can reduce rework, increase productivity, and help ensure that coatings are applied correctly. The importance of proper use of the WFT gauge at the coater to achieve the desired dry film thickness cannot be overemphasized.
