Watchmarks Detailing

Surface · Non-metal panels (plastic and fiberglass)

Fiberglass panels: measuring paint without metal underneath

Paint on fiberglass cannot be read by a magnetic or eddy-current gauge. DeFelsko measures it with an ultrasonic gauge and couplant gel.

Also listed as fiberglass, GRP panels

By Watchmarks EditorialUpdated

Short answer

Paint on fiberglass cannot be read by a magnetic or eddy-current gauge because there is no metal under it. DeFelsko measures coatings over fiberglass with ultrasonic gauges, which need a couplant gel; whether a gauge reports separate layers depends on the model.

Key facts

Material
Fiberglass[1]
Gauge method
Ultrasonic[1][2]
Gauge mode
None: Fe and NFe modes need a metal substrate[3]
What the reading includes
Total coating thickness; layer reporting is model-specific[2]DeFelsko's PosiTector 200 B3 can report up to three layers.
Published thickness figures
Not confirmedNo source we checked gives a typical paint thickness on fiberglass.
Polishing note
Not confirmedNo source we checked gives polishing limits for painted fiberglass.
Inspection tip
Use an ultrasonic gauge with couplant gel; a Fe/NFe gauge will not read[2][1]

Watch out for

  • Same limits as plastic. Fe and NFe gauges need metal under the coating. See non-metal panels.

Questions

Can a paint gauge measure paint on fiberglass?

Only an ultrasonic one. DeFelsko says paint over plastic and fiberglass is measured with ultrasonic gauges such as its PosiTector 200, using a couplant gel.[1][2]

Sources

  1. Automotive Paint Inspection - Paint Meters · DeFelsko Corporation · DeFelsko article on used-vehicle paint inspection: measurement principles by substrate, inspection method, and a rust-repair case study (PosiTest DFT Combo). · checked 2026-10-07
  2. How to Use Paint Thickness Gauges for Better Automotive Detailing · DeFelsko Corporation · DeFelsko detailing guide: what paint gauges measure, typical factory film builds, F/N display codes, units, PosiTest DFT, PosiTector 6000 and PosiTector 200 options. · checked 2026-10-07
  3. Coating Thickness Gauges FAQ · UNI-TREND Technology (UNI-T) · UNI-T FAQ on what coating thickness gauges measure and what FE/NFE mean. · checked 2026-10-07

Next review due 2027-01-07.