How does a coating thickness gauge work and how are they used to value classic cars?

A coating thickness gauge, also known as a paint meter or paint thickness gauge, measures the total thickness of paint and other coatings. It determines the distance between the probe and the base material, including all layers of paint, primer, and any additional materials, such as filler. Common sensor technologies used include magnetic induction, eddy current induction, ultrasonic, and magnetic pull-off.

In industrial settings, coating thickness measurements are utilized for process and quality control purposes. Automotive finishes are complex multi-layer structures (Figure 1). With classic cars, coating thickness gauges are important tools for appraisals and evaluations. They are used to identify if a body panel has been repainted multiple times or has undergone repairs.

Figure 1. Automotive finishes consist of multiple layers of different materials and thicknesses. (Image: New Old Cars)

Magnetic induction

There are two types of magnetic induction coating thickness meters. One type uses a permanent magnet and the other uses an electromagnet.

Permanent magnet designs utilize a Hall-effect sensor or a magneto-resistor, such as anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), or tunneling magnetoresistance (TMR) device, to sense the magnetic flux density and calculate the coating thickness.

Electromagnetic induction designs use two coils, an exciter and a receiver, usually wound on a common core, to measure changes in magnetic flux density in a coating and calculate the thickness of non-magnetic coatings on magnetic substrates. They use low-frequency alternating currents, typically in the low Hz range.

Magnetic induction tends to be less accurate than eddy current coating thickness gauges.

Eddy current induction

Eddy current induction meters use high-frequency (>1 MHz) fields to measure the thickness of coatings on non-ferrous metal substrates. The substrate material and the thickness of the coating affect the magnitude of the eddy currents.

These meters can be sensitive to surface roughness, curvature, substrate thickness, type of metal substrate, and distance from an edge. A typical eddy current meter can measure coating thicknesses up to 25 mils (625 micrometers), but some can measure up to 80 mils (2000 micrometers). Accuracy can vary from 10% for lower values to about 4% for higher values.

Dual-mode paint thickness meters that combine magnetic and eddy current technologies are commonly used in painting and powder coating operations. By switching the probe, they are automatically switched between modes, enabling a single meter to measure coating thicknesses on both ferrous and non-ferrous substrates (Figure 2). That can be particularly important with classic cars, as some have metal body panels, while others have lighter-weight aluminum, plastic, or fiberglass panels.

Figure 2. This coating thickness gauge uses magnetic induction or eddy current measurements, depending on the type of probe attached. (Image: The Track Ahead)

Ultrasonic

Ultrasonic coating thickness gauges are used on metallic and non-metallic substrates, such as plastics, composites, or wood. They use an ultrasonic transducer to send a pulse through the coating. Coating thickness is calculated based on the time it takes for the reflected sound waves to return to the meter.

More sophisticated designs can measure the thicknesses of individual layers in a multi-layer system. When sound waves encounter a change in acoustic impedance at the layer interfaces, a portion of the acoustic energy is reflected. By analyzing the acoustic signature of the echo, it’s possible to determine the thickness of each layer (Figure 3).

Figure 3. Ultrasonic gauges can measure the thicknesses of individual coating layers. (Image: BYK)

Magnetic pull-off

The coating thickness on a magnetic substrate, such as a car body panel, can also be measured using a permanent magnet and a calibrated spring. Depending on the thickness of the coating, the magnet is more or less strongly pulled toward the metal substrate. This causes compression in the spring, which moves a dial indicating the coating thickness.

Magnetic pull-off gauges are simple and relatively inexpensive. They have typical tolerances of ±5% and can quickly detect repairs and damage.

Summary

Coating thickness gauges are essential for industrial processes and quality control, as well as for evaluating the condition and value of classic cars. There are a variety of technologies employed depending on the substrate and coating compositions. Some simply measure the overall thickness of all the applied coatings, while other technologies can quantify the thickness of individual coating layers.