KTA-Tator, Inc.

Challenge 2 is a microphotograph (30x, taken in the KTA laboratory with a state of the art, digital microscope) of the impressions made by a wet film thickness gage in freshly applied green primer. As shown in Photograph #1, the wet film thickness gage is pressed firmly through the coating to the substrate as soon as the coating is applied. The gage contains 4 faces; each face provides a different range of thickness. The end points of each face are on the same plane. The steps in between the end points are at fixed distances from that plane. The distance between each step and the plane created by the end points is shown on the gage in mils (microns are also available).

In this case of the Challenge, the substrate was blast cleaned steel. The fact that the end points penetrated through the coating to the steel can be seen by the impression on the left side of the Challenge microphotograph. The impression was created by one of the end points - small specks of bare steel are visible.

Each of the steps that touch the film will also leave an impression in the coating. One of these impressions is shown on the right side of the Challenge microphotograph. In this case, the substrate is not exposed because the step did not completely penetrate the film. When using the instrument, the steps of the gage can also be examined for the presence of paint to determine which steps contacted the coating.

The wet film thickness of the applied coating is determined as being between the last step that touches the film and the adjacent higher step that does not touch the film. Looking closely at Photograph #2, you can see that both end points have made an impression in the film as have the 2 ½ and 3 mil steps, whereas the 3 ½ mil step did not. The wet film thickness is between 3 and 3 ½ mils.

The wet film thickness gage is used by the coating applicator as part of the Quality Control process. Spot checks with a wet film thickness gage help to determine how much coating is being applied, and can minimize or eliminate rework after the coating dries due to an insufficient or excessive film build. Different ranges of wet film thickness gages are distributed by KTA, including versions that are disposable.

The Winner: The responses included pits filled with spot welds, spitting from a spray gun, clumping of epoxy powder coating, touch up of both liquid applied and powder coating, cratering, residue remaining under the coating from penetrant inspection or oil contamination, handling damage, abrasive embedment, microbiological induced corrosion, and even a foot print. Only one person correctly identified the Challenge as being the marks left by a wet film thickness gage. Congratulations to Pete Blattner of Alaron Corporation for the correct answer.