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Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2004

Measuring Paint Coating Dry Film Thickness and Wet Film Thickness

Q. Is there a tool or device that can accurately measure what the DFT will be while being applied in the wet coat application. We run wet paint coat application with high solids paint in multiple colors. I would like to test film thickness prior to the curing process.

I'm not aware of any such device. All of the dry film thickness instruments require the coating to be fully cured before thre instrument comes into contact with the coating. Since there is some shrinkage that takes place while the coating is in the oven you would need to know ahead of time what degree of shrinkage will take place.

Here is what I would do: Take a few steel panels (such as Q-panels) and apply the coating in your usual manner. Immediately after application measure the wet film thickness. Allow the panels to go through the oven and when the coating is fully cured and hard through measure the dry film thickness. Plot a graph that compares the wet film thickness with the final dry film thickness. This now becomes your calibration chart. The more panels you use in your calibration test, the more statistically accurate will be your calibration chart. BUT here is a warning! The calibration chart will only be useful IF the volume solids of the coating is constant. If your painters add thinners to the coating, and the volume solids changes from one day to the next, or from one batch of paint to the next, then the chart will not be accurate. Of-course, you can plot separate calibration tests for a range of volume solids.

Finally, I am assuming that the spray guns are set to the same parameters from one day to the next. If the atomizing air pressures vary, then the amount of solvents that evaporate during the spray application will affect the wet film thickness measurements.

I hope this information is useful.

Best Wishes,

Ron Joseph

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