Wood Finish Thickness
Updated November 2008
DeFelsko manufactures hand-held, non-destructive ultrasonic coating
thickness gages that are ideal for non-destructively measuring
the dry film thickness of wood finishes and other coatings
on wood and wood products. Many industries now use this nondestructive
technology in their quality programs.
models are ideal for wood coatings.
The PosiTector 200 B1 (Standard model) is the economical
and most common solution for measuring the TOTAL coating
The PosiTector 200 B3
(Advanced model) is capable
of measuring both TOTAL coating thickness AND up to 3 individual
layer thicknesses in a multi-layer system. It also features
a graphic readout
analysis of the coating system.
- Total thickness of coatings
- Lacquer on guitars
- Veneers and laminated composites
- Rough coating surfaces
- Powder coating on MDF
- Thick, polyester-based polymers
- Dry Adhesive
- Individual Layer Thicknesses
in a Multi-layer Application
- Other applications
- Coatings with large, solid
- Wood Stains
Measuring total thickness
Some wood finishes are applied
in a number of layers to achieve their desired objective.
Our PosiTector 200 B1 is the ideal solution when applicators
only need to know the final, total thickness of the coating.
The PosiTector 200 is ready to measure most wood finish
applications right-out-of-the-box. It has a measuring range
13 to 1000 microns (0.5 to 40 mils) and is ideal for measuring
lacquers, primers, UV cured coatings, acrylics, powder coatings
and other polymer coatings. It requires no calibration adjustment
for most applications, it is mils/microns switchable, and
has a large, thick impact resistant Lexan display.
For those familiar with magnetic coating thickness gages, using
ultrasonic coating thickness gages is easy and intuitive.
The measurement method is simple
#2: Measuring lacquer on guitars
A good example of how our PosiTector 200 B1 measures
the total thickness of a
multi-layer application can be found in the manufacturing
process of musical instruments.
Lacquer is applied to a guitar substrate
(typically wood or graphite) to attain a glass-like sheen
while showing the beauty and luster of the wood. The
tough durable finish of the lacquer protects the wood from
moisture, chemicals and marring, while optimizing the acoustic
resonance of the instrument.
The method and benefits of ultrasonic thickness testing
for guitars is discussed in a separate
#3: Measuring on veneers and laminated
Thickness measurement is not limited to natural wood. Some
of today’s modern materials are a combination of synthetic
and natural materials.
In woodworking, veneer refers to thin slices of wood that
are usually glued and pressed onto core panels (typically,
wood, particle board or medium density fiberboard) to produce
flat panels such as doors, tops and side panels for cabinets,
parquet floors and parts of furniture. Composite materials
with a wood veneer surface are increasingly used in aerospace,
underwater, and automotive structures.
The wood or wood product surface is often finished with
a polymer coating, the total
thickness of which can easily be measured with a PosiTector
Measuring on Rough Coating Surfaces
Coatings with rough surfaces challenge any measurement method,
and ultrasonic testing is no exception. The PosiTector 200
is equipped to handle these situations.
At a microscopic level, thickness can vary (see Fig.1).
Meaningful thickness measurements are best obtained by taking
several measurements in the same general location and averaging
A plywood coating with surface roughness.
Couplant fills the voids between the probe
and the coating.
On rough surfaces the PosiTector 200 typically identifies
the thickness from the top of the coating peaks down to the
substrate. This is represented by distance #1 in the Fig.2.
Couplant fills the voids between the probe and the coating
to assist the ultrasonic pulse enter the coating.
Severe roughness can cause the gage to display low thickness
values (distance #2). This happens because echoes from the
couplant/coating interface are stronger than the coating/substrate
interface. The PosiTector 200 has a unique user-adjustable SET RANGE
feature to ignore roughness echoes.
To obtain meaningful thickness measurements on rough surfaces,
the memory mode of the PosiTector
200 (Fig.3) is used to calculate a running average.
#5: Measuring powder coating on MDF
Medium density fiberboard, or MDF, is an engineered wood
product made of particles of wood mixed with a synthetic
resin. Powder coating is a dry finishing process, using finely
ground particles of pigment and resin, which are electrostatically
charged and sprayed onto the MDF. The charged particles adhere
until melted and fused into a tough, even coating through
the application of heat and energy.
MDF is suitable for powder coating because of its low porosity
and homogeneous surface. MDF products include office furniture,
kitchen and bath cabinets, doors, store fixtures and displays,
and ready-to-assemble furniture for the office and home.
These single coat applications typically range in thickness
from 3 to 9 mils and are easily measured with a PosiTector
200 B1. For textured surfaces shown above, PosiTector
200 has a unique user-adjustable SET RANGE feature to ignore
roughness echoes. And the instrument’s memory
mode is used to display the average of a series of measurement
results. The large LCD also displays the number of readings
taken, the standard deviation, and the highest and lowest
In this example, 10 measurements have been
The last measurement of 18.2 mils is displayed along
deviation and max/min values of all 10 readings.
The average value is 18.26 mils.
#6: Thick, Polyester-based polymers
Polymer coatings are applied to wood based materials such
as MDF (medium density fiberboard), plywood, chipboard and
particleboard to provide an aesthetic durable finish. Entire
industries have been developed around the use of polymer
coatings. For example, the polyester based Granicoat® (Fig.5)
is sprayed on to wood surfaces such as portable buffet tables,
kitchen countertops, bath vanities, as well as desk and bar
Measuring Granicoat® thickness.
For these thick coating applications we recommend our PosiTector
200 C3. It has a measuring range of 50 to 3800 microns
(2 to 150 mils) and is ideal for thicker coatings. Its graphics
mode is useful in determining the thickness of thin primers
and other hard-to-distinguish layers.
Using the cursor.
Figure 6 shows what would appear on
LCD when a measurement is taken when graphics mode is turned
on. Both the thickness values and a graphical
representation of echo “spikes”, "peaks" are displayed.
Interestingly, a smaller peak can also be seen. Although
it is too faint to measure automatically, a green cursor can be
positioned over top for manual measurement.
The green cursor in figure 7 tells us
the indicated peak is 4.4 mils thick.
#7: Dry Adhesive
There are many types of wood adhesives. Wet adhesives cannot
be measured ultrasonically because the probe of the PosiTector
200 must touch the surface of the material being measured.
However, dry adhesive is much like any other single
layer coating and can be measured by the PosiTector 200
#8: Measuring the Individual Layer
Thicknesses in a Multi-layer Application
The PosiTector 200 B3 is capable of measuring BOTH the
total coating system thickness AND up to
3 individual layer
thicknesses in a multi-layer system. It also features a graphic
readout for detailed analysis of the coating system.
In the above 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. A two-step process adjusts
the instrument for multi-layer applications.
Other applications ideally suited for ultrasonic coating
UV cured coating on hickory
Water borne acrylic primer on plywood
Factory primer on untextured molded hardboard
UV cured coating on unfilled maple
Roll and curtain coated acrylic filler/base/top coats
Sprayed lacquer on oak
Application A: Coatings with large, solid particles
The probe of the PosiTector 200 contains an ultrasonic transducer
that sends an ultrasonic vibration into the coating. This
vibration travels through the coating until it encounters
a material with different mechanical properties - typically
the substrate. The vibration is partially reflected at this
interface and propagates back to the transducer.
For best accuracy, the ultrasonic vibration must proceed
unimpeded through the coating. Large solid particles such
as sand deflect the vibration and prevent a clear echo from
returning to the probe. Therefore the PosiTector 200 is unable
to repeatably measure these applications.
B: Wood Stains
A wood stain is a sub-category of paint. It consists of
a pigment suspended in a "vehicle" of solvent and
binding agent (alkyd, linseed oil, acrylic, polyurethane,
lacquer, or resin). It is formulated to impart or transport
the pigment into the pores of the surface rather than creating
a film on top of the surface.
The PosiTector 200 is designed to measure the thickness
of films built-up on the surface of wood. Stains, which penetrate
deep into wood fibers to seal and protect wood, cannot be
How to Measure
Ultrasonic testing measurement works by
sending an ultrasonic vibration into a coating using a probe
(i.e. a transducer) with the assistance of a couplant applied
to the surface. A 4 oz bottle of a common water-based glycol
gel is included with
every instrument. Alternatively, a drop of water can serve
as couplant on smooth, horizontal surfaces.
Taking a measurement.
After the drop of couplant has been applied to the surface
of the coated part, the probe is placed flat on the surface.
Pressing down initiates a measurement (see Fig.10). Lifting
the probe when a double beep is heard holds the last measurement on the LCD. A second
reading may be taken at the same spot by continuing to hold
the probe down on the surface. When finished, wipe the probe
and the surface clean with a tissue or soft cloth.
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.
Graphics Mode (PosiTector 200
- Advanced models only)
The right hand side of the PosiTector 200’s screen
can be used to display a graphical representation of the
ultrasonic pulse as it passes through the coating system.
This powerful tool enables the user to better understand
what the gage “sees” below the surface of the
PosiTector 200 B3 with Memory Mode ON
Left: PosiTector 200 B3 with Graphics Mode ON
Right: PosiTector 200 B3 with Graphics Mode OFF
As the probe is depressed and the ultrasonic pulse travels
through the coating system, the pulse encounters changes
in density at the interfaces between coating layers and between
the coating and the substrate.
A “peak” depicts these interfaces. The greater
the change in density, the higher the peak. The more gradual
the change in density, the greater the width of the peak.
For example, two coatings layers made of essentially the
same material and "blended" would result in a low,
wide peak. Two materials of very different density and a
well-defined interface would result in a high, narrow peak.
The PosiTector 200 B3 chooses the
highest of peaks when trying to determine coating layer
thickness. For example, if the number of layers is set
to 3, the 3 highest peaks between the Lo & Hi Set Range are
selected as the interfaces between these layers. The peaks
that the Gage selected are indicated by red triangle
arrows (see Fig.12).
In Fig. 12, the left (Lo - 1.9mils) and right (Hi = 11.0 mils)
Range values are displayed as two numbers at the
top left and top right of the graphics display screen. Lo (the minimum
limit) is at the left. Hi (the maximum limit) is at the
right. Echoes or peaks (thickness values) outside these
ranges are ignored. Range values are set and modified using
the Set Range menu option.
This Graphics display can be manipulated with the Set Range
menu option. In addition to being able to adjust the Lo & Hi Range
values, a green Cursor can be positioned anywhere between the set range values to investigate other peaks.
A green cursor is shown.
Conventional magnetic and eddy-current gages only work on
metals. Since these instruments can' t measure the thickness
of finishes over wood, alternate techniques have been used
- 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.
High-powered magnification reveals surface roughness of both
the coating and the wood substrate.
A typical destructive technique requires cutting the coated
part in a cross section and measuring the film thickness
by microscopically viewing the cut. Another cross sectioning
technique uses a scaled microscope to view a geometric incision
through the dry-film coating. To do this, a special cutting
tool makes a small, precise V-groove through the coating
and into the substrate (see Fig.15). Gages are available
that come complete with cutting tips and illuminated scaled
magnifiers. A detailed description of this test method is
provided in ASTM
D4138-07a, “Standard Practice for Measurement of
Dry Film Thickness of Protective Coating Systems by Destructive,
Although this method' s principles are easy to understand,
opportunities abound for introducing errors. It takes skill
to prepare the sample and interpret the results. Also, adjusting
the measurement reticule to a jagged or indistinct interface
can generate inaccuracy, particularly between different operators.
This method is used when inexpensive, nondestructive methods
aren't possible, or as a means of confirming nondestructive
With the arrival of ultrasonic instruments, many finishers
have switched to non-destructive inspection.
Background on Wood Coatings
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 fiberboard
(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
The PosiTector 200 B3 can calculate the average and standard
of a series of measurements for each layer in a
Why measure with Ultrasonics?
Manufacturers and applicators alike have long believed that
there is no simple and reliable means for non-destructively
measuring coatings on wood and wood products. Their common
solution was to place metal coupons next to the part and
then measure the thickness applied to the coupon with either
a mechanical or electronic gage -- magnetic or eddy current.
This labor intensive solution is based on the assumption
that a flat coupon placed in the general coating area receives
the same paint profile as the wood part in question. An ultrasonic
solution enables the user to measure the total coating thickness
of the actual part. Dependent on the ultrasonic gage utilized
and the coating application process, an added advantage is
the ability to identify multiple distinct layers.
Ultrasonic coating thickness measurement is now an accepted
and reliable testing routine used in wood industries. The
standard test method is described in ASTM D6132-08. “Standard
Test Method for Nondestructive Measurement of Dry Film Thickness
of Applied Organic Coatings Using an Ultrasonic Gage” (2008,
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
Couplant is required to propagate ultrasound into the coating.
Water is a good couplant for smooth coatings. Use the supplied
glycol gel for rougher coatings. While it is unlikely that
the couplant will damage the finish or leave a stain on the
surface, we suggest testing the surface by using the couplant
on a sample. If testing indicates that staining has occurred,
a small amount of water can be used instead of couplant.
Consult the Material Safety Data Sheet available on our website
and your coating supplier if you suspect the couplant may
damage the coating. Other liquids such as liquid soap may
also be used.
PosiTector 200 Standard models can record 250 measurements. PosiTector 200 Advanced models can store 100,000 measurements in up to
1000 batches for on-screen statistical purposes,
for printing to an optional
Bluetooth Wireless Printer, or for downloading to a personal computer
using the provided USB Cable and one of the PosiSoft Solutions.