Fred is right about the multi-layer ultrasonic units (e.g. PosiTector 200) - it's doesn't work on metal. Here the response I got from DeFelsko:
"Our PosiTector 200 B-Adv is designed for measuring the thickness of paint applied to non-metal substrates. It is not designed for measuring the thickness of paint applied to metal substrates."
"Our PosiTector 200 B-Adv is designed for measuring the thickness of paint applied to non-metal substrates. It is not designed for measuring the thickness of paint applied to metal substrates."