How can we measure the thickness of paint once cured?

Q. We manufacture plastic parts that are coated with a paint material. How can we measure the thickness of paint once cured?—Z.Y.

A. There are a couple of ways to determine the mil thickness of paint on any substrate surface: destructive and non-destructive. A mil is a thousandth of an inch or 0.001”, and in the paint or coatings industry it is the standard measurement used to determine the thickness of coatings material, wet or dry.

There are two common “non-destructive” methods for determining mil thickness on a cured part. The first is by using an ultrasound device; a gel is applied to the surface, then a stylus probe is passed over it to reveal a digital mil-thickness readout. The second non-destructive method is by using an old-school standard digital micrometer. To test using this method, first measure the thickness of the part before it is coated, then measure again after it has been painted and cured. The difference in the reading would be the mil thickness of application.  This technique really works well if the part surface is flat.

In the lab, I’ve relied on a more “destructive” method using a Tooke paint inspection gage, a very precise tool used for ASTM D4138 testing and that can be used on both single and multiple coats. In this method, you first make a predetermined 1-, 2- or 10-mil V-shaped cut all the way down to the substrate, then view the incision through a 50x reticle scale lens. When lined up correctly over the incision, this lens will help you to determine the actual coating thickness. Although this method is destructive, it is highly accurate.

Ultrasound devices, digital micrometers and Tooke tools can be purchased from any reputable lab testing equipment manufacturer.