Coating thickness gages that use ultrasonic measurement techniques are becoming increasingly popular. They support or replace destructive methods for measuring the thickness of coatings over wood and wood products.

Coatings serve a variety of functions. Some are designed to restore, protect, waterproof, and beautify wood structures. Others are specifically formulated to seal and fill pores and to provide an aesthetically pleasing surface texture. Penetrating finishes are absorbed into the wood and harden to create a strong protective barrier that will not flake off.

Why Measure Thickness?

Coatings are designed to perform their intended function best when applied within a tight thickness range as specified by the manufacturer. For example, conversion varnishes are harder than other coatings and should not be used in excess of 5 mils dry thickness in order to prevent cracking or other finish failures. Nitrocellulose lacquer should usually be kept lower than 3 mils. A consistent mil thickness is paramount when applying lacquer base coats and crack coats to achieve a desired crackle finishing effect.

On medium density fibreboard (MDF), powder coating thickness typically ranges between 3 to 9 mils. Usually the thicker the mil coverage, the more durable the finish. Factory specifications often call for a stated ±1 mil tolerance. This level of quality cannot be determined just by looking at it.

There are other benefits to precisely measuring finish thickness.  When companies fail to check and verify coating quality of incoming material, they waste money reworking product. By checking spray operators technique they ensure the coating is being applied in compliance with the manufacturers’ recommendations. Besides, applying excessive film thickness can drastically reduce overall efficiency. Finally, regular testing can reduce the number of internal reworks and customer returns due to finishing defects.

How Best to Test?
                                                                                                                                                   
Over metal, testing the thickness of coatings is commonplace for quality control and inspection purposes. When the base metal is carbon steel, a magnetic method is used. Eddy current devices are used for the other metals such as copper and aluminum.

Since these instruments can' t measure the thickness of finishes over wood, alternate techniques have been used including:

• Optical cross-sectioning (cutting the coated part and viewing the cut microscopically)
• Height measurement (measuring before and after with a micrometer)
• Gravimetric (measuring the mass and area of the coating to calculate thickness)
• Dipping wet film thickness gages into wet paint and calculating dry-film thickness using the percent of solids by volume
• Substitution (placing a steel coupon alongside the wood part and coating it at the same time).

These tests are time-consuming, difficult to perform, and are subject to operator interpretation and other measurement errors. Applicators find destructive methods impractical. To get a statistically representative sample, several wood products from a lot might need to be scrapped as part of the destructive testing process.

With the arrival of ultrasonic instruments, many finishers have switched to non-destructive inspection.

Ultrasonic Breakthrough

Quality professionals are already familiar with various aspects of ultrasonic testing wherein high-frequency sound energy is used to conduct examinations and make measurements. Ultrasonic testing can detect and evaluate flaws in metal, measure dimensions, ascertain material characterization and more.

Wall-thickness measurement is perhaps the most common and simple of ultrasonic tests. Precision ultrasonic wall-thickness gages permit quick thickness measurement of objects without requiring access to both sides. For coating measurement, however, these gages are not ideal. They do not have sufficient sensitivity to measure the thickness of acrylic fillers, factory primers, lacquers, UV finishes, powder coatings and other materials used over wood.

Figure 1
The PosiTector 200 ultrasonic
coating thickness gage.

The first hand-held instrument designed specifically for coating thickness measurement appeared on the market 14 years ago and is now into its fourth generation. It uses a single-element transducer and advanced numerical techniques to filter and enhance digitized echoes. Today’s hand-held ultrasonic coating thickness gages are simple to operate, affordable, and reliable (Figure 1).

A Sound Measurement Technique

Because a potentially large number of echoes could occur, the gage is designed to select the maximum or “loudest” echo from which to calculate a thickness measurement. Instruments that measure individual layers in a multi-layer application also favor the loudest echoes. The user simply enters the number of layers to measure, say three, and the gage measures the three loudest echoes. The gage ignores softer echoes from coating imperfections and substrate layers.

Measurement Accuracy

The accuracy of any ultrasonic measurement directly corresponds to the sound velocity of the finish being measured. Because ultrasonic instruments measure the transit time of an ultrasonic pulse, they must be calibrated for the “speed of sound” in that particular material.

From a practical standpoint, sound velocity values do not vary greatly among the coating materials used in the wood industry. Therefore, ultrasonic coating thickness gages usually require no adjustment to factory calibration settings.

Where the Coating meets the Substrate

A factor influencing the accuracy and repeatability of ultrasonic measurement is how these coatings interface with the wood substrate. Figure 3 shows two examples of coated wood. These photos, taken at higher resolution than most field destructive tests are capable of, clearly show the boundary between the finish and the wood. The finish coating may look smooth on top, but thickness may be inconsistent. Wood substrates often are grainy with varying degrees of surface roughness and primer penetration. Such porosity and roughness may promote adhesion but they increase the difficulty of attaining repeatable thickness measurements by any means.

Figure 3
Two examples of uneven coating/substrate regions.

Ultrasonic gages are designed to average small irregularities to produce a meaningful result. On particularly rough surfaces or substrates where individual readings may not seem repeatable, comparing a series of averaged results often provides acceptable repeatability.

Figure 4
Some instruments measure the individual layers in a multi-layer system. 
In this example, layer 1 is 1.5 mils thick. Layer 2 is 1.5 mils thick.
Total thickness is 3.0 mils.  The graphical LCD displays
two “peaks” representing two material interfaces.

Ensuring the Right Sound

Ultrasonic testing brings distinct benefits to the wood industry. Furniture, flooring, and musical instrument manufacturers often apply several layers of lacquer or similar finishing materials. Some processes require the ability to identify the thickness of individual layers or series of layers. When applied at the wrong thickness, the coating layer that beautifies and protects a high-quality guitar, for example, can easily detract from its sound. Too much coating can dampen the guitar’s acoustic resonance; too little can have the reverse effect.

Figure 5
Non-destructive measurement of lacquer
thickness on wood guitars.

Musical instrument manufacturers now use ultrasonic gages to accurately and non-destructively measure the lacquer on their valuable products. As a result of using this new technology they have not only decreased their lacquer usage, but they are able to take meaningful measurements without excessively disrupting their production process. There' s no need to scrap product to measure a coating thickness, and the thickness over the entire surface can easily be measured to ensure a smooth, even coating. Rework is minimized due to an increased ability to control the coating process.

Figure 6
Some instruments provide statistical analysis.
In this example, 10 measurements have been taken.
 The last measurement of 18.2 mils is displayed
along with the average, standard deviation and max/min values of all 10 readings.

Figure 7
These instruments are simple to operate,
affordable and reliable.

A Final Echo

Ultrasonic coating thickness measurement is now an accepted and reliable testing routine used in wood industries. The standard test method is described in ASTM D6132-04. “Standard Test Method for Nondestructive Measurement of Dry Film Thickness of Applied Organic Coatings Using an Ultrasonic Gage” (2004, ASTM). To verify gage calibration, epoxy coated thickness standards are available with certification traceable to national standards organizations.

Quick, non-destructive thickness measurements can now be taken on materials that previously required destructive testing or lab analysis. This new technology improves consistency and throughput in the finishing room. Potential cost reductions include:

• Minimizing waste from over coating by controlling the thickness of the coating being applied
• Minimizing rework and repair through direct feedback to the operator and improved process control
• Eliminating the need to destroy or repair objects by taking destructive coating thickness measurements

Today, these instruments are simple to operate, affordable and reliable.

DAVID BEAMISH is President of DeFelsko Corporation, a New York-based manufacturer of hand-held coating test instruments sold worldwide. He has a degree in Civil Engineering and has more than 25 years of experience in the design, manufacture, and marketing of these testing instruments in a variety of international industries including industrial painting, quality inspection, and manufacturing. He conducts training seminars and is an active member of various organizations including NACE, SSPC, ASTM and ISO.