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How do I use a wet film thickness (WFT) gauge?

April 2011 Week One – Coatings Management

 

How do I use a wet film thickness (WFT) gauge?

 

A wet film thickness gauge is designed to give the installer immediate feedback as to the film builds applied. 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 application. 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 gauge being used is the notched gauge (see figure 1). Others types including the eccentric disk, the rolling notch gauge and the 6 sided gauges are available from specialty vendors.

Figure 1

            

 

 

 

 

 

 

There are several issues that must be addressed when using a WFT gauge.

1. Technique

2. Timing

3. Creating surface defects

 

Technique

When placing the gauge on a freshly painted surface, the gauge must be placed 90 degrees to the surface so the card is upright in your hand.

The installer 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 surface (see figure 2 below) 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).

Figure 2

 

 

 

 

 

 

 

 

Timing

The solvent in a coating will immediately start to evaporate after application. 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 application. If another installer measures the thickness after 20 seconds, the results would be different even if the initial thickness was identical.

 

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).

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