How do I use a wet film thickness (WFT) gauge?A wet film thickness gauge is designed to give the spray operator immediate feedback as to the film build just sprayed. In most cases, measuring the dry film thickness (DFT) provides little information as it is usually measured a considerable amount of time after the actual spraying. Many things could have influenced the DFT: operator fatigue, ambient air temperature, coating temperature, etc.There are several types of WFT gauges available. The most common being the notch gauge (see figure 1). Others types including the eccentric disk, the rolling notch gauge and the 6 sided gauges are available from specialty vendors. There are several issues that must be addressed when using a WFT gauge.
Technique
Timing
Reading with clear coats
Creating surface defects
Technique
When placing the gauge on a freshly painted part, the gauge must be placed 90 degrees to the part. The operator also needs to be aware of variation of the surface that may influence the reading. For example, if the surface is not perfectly flat, one direction may give a more accurate reading than another.
To use the WFT gauge, place the gauge directly on the wet finished part (see figure 2) and as described above. The notches will indicate the measured film thickness. For example, if the 1 and 2 mil notches are wet and the 3 and 4 notches are dry, then the measured thickness is between 2 and 3 mils (.002 to .003 inches)
Reading with Clear Coats
A clear coating on a WFT gauge would be very difficult to read. The most common method of reading clear coats is to use the gauge as a stamp on a piece of absorbent (non-gloss) paper. Many companies use the stamp method as a way of documenting the WFT.
Creating Surface Defects
After using a WFT gauge to check the film thickness, the material may not flow to hide the area where the gauge was used. If this creates an undesirable defect, place a small sample of the material in line with the operators normal spray path. This sample should be sprayed along with the part. The sample then may be checked for WFT or DFT (after curing).
