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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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