If the thickness of a metal substrate is tightly controlled (very important qualifier), thickness of a nonconductive coating can be monitored/measured with capacitive sensors.
After the coating is applied, the bottom of the metal substrate is precisely located on a reference surface. A capacitive probe mounted over the coated material will respond to changes in the coating thickness. It will also respond, with greater sensitivity, to changes in thickness of the substate material which is why the substrate thickness must be precisely controlled.
For more information on measuring nonconductive coatings on metal substrates while compensating for changes in substrate thickness, see our Application Note: Dual Technology Film Thickness Measurement.