Paint film thickness is one of the core indicators for evaluating the quality and performance of coatings, which is directly related to the protective ability, service life and appearance effect of the product. In laboratory testing and on-site quality control, accurate measurement of paint film thickness is a critical step in ensuring that coatings meet design specifications and standards. For different substrates, coating structures and measurement accuracy requirements, the industry has developed a variety of measuring instruments and methods based on different physical principles. This article aims to systematically sort out the technical principles, applicable scenarios and related standards of various paint film thickness measuring instruments, and provide reference for professional and technical personnel.
Overview of measurement method classification
According to the measurement principle and coating state, the measurement method of paint film thickness can be mainly divided into two categories: dry film thickness measurement and wet film thickness measurement. Dry film thickness measurement is the final basis for coating curing, and non-destructive instrumental measurement methods have become the mainstream for laboratory and field applications due to their fast and non-destructive characteristics. For multi-coating systems or special substrates, layered measurements need to be carried out with the help of microscopy or ultrasonic methods.
Mainstream dry film thickness measurement instrument
At present, the most widely used dry film thickness measuring instruments in laboratories and industrial fields are mainly based on the three principles of magnetic induction, eddy current and ultrasound. Each technique has its own specific boundaries of application, and the choice of instrument depends on the material of the substrate (e.g., ferromagnetic, non-ferromagnetic, or non-metallic).
Magnetic induction thickness gauge
This technology is suitable for measuring the thickness of non-magnetic coatings (such as paint, galvanized, epoxy coatings, etc.) on magnetic metal substrates such as steel. The principle is based on a change in magnetic flux or magnetic attraction between the probe and the magnetic substrate, which is proportional to the thickness of the coating. This method has high precision and good stability, and is the most commonly used method for laboratory testing of steel structure anti-corrosion coatings.
Eddy current thickness gauge
When the substrate is non-ferromagnetic metals such as aluminum, copper and their alloys, eddy current thickness gauges become a reliable choice. The high-frequency coil in the instrument probe generates an alternating electromagnetic field, which generates an eddy current on the metal substrate below, and the influence of this eddy current on the probe coil varies with the coating thickness (i.e., the distance between the probe and the substrate). This method is also non-destructive and fast in measurement, and is suitable for paint film detection on automobile bodies and aluminum alloy components.
Ultrasonic thickness gauge
The advent of ultrasonic technology has pushed through the limitation that the substrate must be metal. According to GB/T 37361-2019 "Ultrasonic Thickness Gauge Method for Determination of Paint Film Thickness" and ISO 19397:2024, the method emits ultrasonic pulses to the coating through a probe, which penetrates the coating and reaches the substrate interface and reflects back, and calculates the thickness by accurately measuring the round-trip time of sound waves in the coating. This method is not only suitable for coating measurement on non-metallic substrates such as plastics, wood, glass, ceramics, etc., and has the ability to measure the thickness of each layer in a multi-layer coating system.
Microscopy
For multi-coated systems or complex samples that cannot be directly measured by the instruments mentioned above, microscopy is considered a means of arbitration. This method requires cutting the sample from the measured workpiece, inlaying, grinding and polishing to make a metallographic sample, and then directly observe the cross-section under a metallographic microscope or electron microscope and measure the thickness of each layer. Although this method is destructive and the sample preparation process is relatively cumbersome, it can provide intuitive and accurate data on the thickness of each layer, and is often used for measurement uncertainty assessment and method validation.
| Instrument category | Technical characteristics and applicability |
|---|---|
| Magnetic induction thickness gauge | Suitable for ferromagnetic metal (steel, iron) substrates and for measuring non-magnetic coatings. High accuracy, less affected by the electrical properties and geometry of the substrate. |
| Eddy current thickness gauge | Suitable for non-ferromagnetic metal (aluminum, copper, alloy) substrates and for measuring non-conductive coatings. It is sensitive to the conductivity of the substrate and the lifting effect of the probe. |
| Ultrasonic thickness gauge | Suitable for almost all substrates (metal, plastic, wood, etc.) and can measure the thickness of single and multiple coats. The measurement requires the use of a couplant, which requires certain operator experience. |
| Microscopy | Destructive testing for multi-layer systems or complex coatings. Samples need to be prepared and observed with the help of a microscope, and the measurement results are intuitive and often used as a reference method. |
Measurement of wet film and powder coating thickness
During the coating process, the measurement of wet film thickness or uncured powder coating thickness has process control significance and can help the operator adjust the process in time to ensure that the final dry film thickness meets expectations.
Wet film thickness comb gauge: This is a simple field measurement tool, usually made of corrosion-resistant materials with multiple backlasks. After pressing it vertically into the wet film, the thickness of the wet film can be roughly estimated by observing the location of the tooth marks that have been infiltrated by the coating. This method is fast and simple, but the accuracy is relatively low, which is affected by the operation technique and the surface tension of the coating.
Powder coating thickness measurement: For thermoset powder coatings, their thickness changes significantly before and after curing due to melt leveling. According to ASTM D7378, when measuring the thickness of an uncured powder layer, a conversion factor for a specific powder is established to predict its cured film thickness. This can be obtained by using a non-contact ultrasonic powder thickness gauge, or using a comb gauge or magnetic thickness gauge to measure the powder layer, and then combine it with the measured thickness at the same point after curing.
Key influencing factors and standardized operations
In order to ensure the accuracy and traceability of the measurement results, the following technical details must be paid attention to during the measurement process and strictly follow relevant domestic and foreign standards. The physical properties of the substrate, especially the surface roughness, have a significant impact on the measurement results. On a rough surface that has been blasted, the magnetic thickness gauge reads the thickness at a point between the crest and the trough, rather than the absolute crest to trough height. Therefore, when measuring thin coatings on rough surfaces, it is often recommended to interpret the data with caution, and it is often recommended to increase the number of measurement points and take a statistical average, or indicate the roughness of the substrate in the report.
Regular calibration and zeroing of the instrument is the basis for ensuring measurement accuracy. Zero point correction and range calibration should be performed on a smooth standard calibration sheet or uncoated clean substrate before use. For ultrasonic thickness gauges, the sound velocity must be set to match the material characteristics of the coating to be measured.
Citation standards and literature
In the process of writing, this article refers to the following domestic and foreign standards and industry technical literature to strive for technical rigor and standardization of the content.
1. National standard information public service platform. GB/T 37361-2019 Determination of paint film thickness -- Ultrasonic thickness gauge method .
2. China Corrosion and Protection Net. Common methods and application standards for coating thickness detection .
3. China Standard Service Network. ISO 19397:2024 Coloured paints and varnishes - Determination of coating film thickness by ultrasonic thickness gauge .
4. Jiayu Testing Network. Eight major paint film performance testing methods and standards .
5. China Corrosion and Protection Net. The relationship between surface roughness and paint film thickness .
6. ANSI Webstore. ASTM D7378-16(2024) Standard Practice for Measurement of Thickness of Applied Coating Powders to Predict Cured Thickness .
7. Afnor EDITIONS. ISO 19397:2024 Paints and varnishes - Determination of the film thickness of coatings using an ultrasonic gauge .