[jaden]

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Yes.
Standard Coating Thickness gauges will measure the fiberglass thickness.

Assuming the fiberglass is properly stuck to the steel an is ‘nothing weird’ then multimode (echo-echo) ultrasonic thickness gauges (not the cheaper single echo only) will give accurate readings.

For poorly adhering or sound insulating coatings then pulsed eddy current should work, but that is much more complex/ expensive.