As seen in issues of:
Finishing Today, Feb. 2008
Materials Performance Magazine,
Quality Digest Magazine, Oct. 2004
Coating Thickness Gages
by David Beamish, DeFelsko Corporation
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
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
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
- 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.
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
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.
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
Ultrasonic testing works
by sending an ultrasonic vibration into a coating using
a probe (transducer) with the assistance of a couplant
applied to the surface.
The vibration travels through
the coating until it encounters a material with
different mechanical properties—typically
the substrate but perhaps a different coating layer.
The vibration, partially reflected at this interface,
travels back to the transducer. Meanwhile, a portion
of the transmitted vibration continues to travel
beyond that interface and experiences further reflections
at any material interfaces it encounters (Figure
Ultrasonic vibrations reflect off
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.
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.
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
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
two “peaks” representing two
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.
Non-destructive measurement of lacquer
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.
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.
These instruments are simple to operate,
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
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
- 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
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.