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
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:
- Reading with clear coats
- Creating surface defects
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
The solvent in a coating will immediately start to
evaporate after spraying. In order to achieve a common method of
reading the coating thickness, a time frame will need to be
established. Typically, one might measure the thickness 5 to 10
seconds after spraying. If another operator measures the thickness
after 20 seconds, the results would be different even if the initial
thickness was identical.
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
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 and DFT (after curing).