Years ago we used a "Toke" Gage to scribe thru the primer to the substrate, and it has a graduated scope to look into and see the measurement of the thickness of the primer in mils. This was a Ford requirement on all our primed parts.This gage was inexpensive, and easy to use. All floor inspectors used it, and recorded the thickness. It may work on powder coatings, if that is what you are refering to.