Paint Thickness Measurement
Updated October 2008
DeFelsko manufactures hand-held, non-destructive ultrasonic coating
thickness gages that are ideal for non-destructively measuring
the dry film thickness of coatings on plastic. Many industries
now use this nondestructive technology in their quality programs.
models are ideal for plastic substrates.
- 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 graphics mode for detailed
analysis of the coating system.
- Total thickness of coatings
- Measuring Rough Coating
- Individual Layer Thicknesses
in a Multi-layer Application
- Measuring on Automotive Plastic
#1 : Measuring total thickness
Some plastic coating systems 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 B1 is ready to measure most plastic
coating applications right-out-of-the-box. It has a measuring
range of 13 to 1000 microns (0.5 to 40 mils) and is ideal
for measuring total coating system thickness. It requires
no calibration adjustment for most applications, is mils/microns
switchable and has a large, thick impact resistant Lexan
For those familiar with magnetic coating thickness gages, using
ultrasonic coating thickness gages is easy and intuitive.
The measurement method is simple
#2: Measuring 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.2). Meaningful thickness measurements are best
obtained by taking several measurements in the same general
location and averaging the results.
Fig.1 Measuring on a textured
fills the voids between the
probe and the coating.
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.
For these applications, the memory
mode of the PosiTector 200 provides assistance. With
memory turned ON, the PosiTector 200 calculates and displays
the number of readings taken, the average of those readings,
the standard deviation of those readings, and the highest
and lowest readings (see Fig.3). The supplied couplant
works better than water on rough surfaces.
Fig.3 To obtain meaningful
thickness measurements on rough surfaces,
the memory mode of
the PosiTector 200 is used to calculate a running average.
#3: Measuring the Individual Layer Thicknesses in a Multi-layer
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.
#4: Measuring on Automotive Plastic
A large number of plastic
components are present both in car interiors and exteriors.
Very often these plastic components have to be coated, for
aesthetic and protection requirements.
Interior components like dashboards, door panels, airbag
covers, steering wheels etc, are more challenging to measure
for two reasons. First, many components are too small or
too geometrically complex for the PosiTector
200’s probe to access properly. Second, some parts
either have a very thin coating or a coating too
rough for the gage to measure consistently. The gage
performs best on a smooth, flat, hard surface with a coating
thickness of at least 13 microns (0.5 mil).
Exterior components like bumpers, mirror shells, side claddings,
etc. can be measured provided the PosiTector
200’s probe can again access the surface to be
measured. The gage can measure the total thickness of most
applications, and can measure some individual layers in a
Automotive coating systems are comprised of several
coating layers. The basic PosiTector 200 B1 model
is capable of measuring the total thickness of these coating
The adhesion promoter and primer layers in automotive applications
are typically too thin for the more advanced PosiTector 200
B3 model to measure individually. So the instrument combines
their thicknesses with the base coat thickness to produce
a total result. The top, clear coat layer is measured separately
Figure 5 is typical of results
from the PosiTector 200 B3 on exterior automotive plastic.
The image on the left shows a measurement using imperial
(mils) units. The right image is the equivalent measurement
in metric (microns). Although the coating system is comprised
of 4 layers, the instrument combines the thicknesses of the
first 3 layers (adhesion promoter, primer, base coats) into
one value of 1.7 mils (43 microns). The final top clear coat
is measured individually as 1.7 mils (43 microns). The total
coating system thickness is 3.4 mils (86 microns).
This result is useful when the final clear coat thickness
is the important layer to determine. Auto
detailers use this feature to view the remaining amount
of clear coat while polishing. Applicators use this feature
to ensure consistent application thickness.
Ultrasonic probes discussed in this
article have an 8 mm (0.3") diameter flat measurement surface that should
fully contact the coated plastic for best measurement results. Measurements
taken on curved surfaces may require averaging of multiple
readings for meaningful results.
How to Measure
Ultrasonic measurement of coating thickness works by
sending an ultrasonic vibration into a coating using a probe
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.
Fig.6 Taking a
After a 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.6). Lifting
the probe when a double beep is heard or when the green indicator
light blinks 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 plastics industry. Therefore, ultrasonic
coating thickness gages usually require no adjustment to
factory calibration settings.
Graphics Mode (PosiTector 200
B3 model 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
Left: Graphics Mode ON
Right: Graphics Mode OFF
7-8 PosiTector 200 B3
with Memory Mode ON
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 Ranges are
selected as the interfaces between these layers. The peaks
that the Gage selected are indicated by red triangle
arrows (see Fig.9).
In Fig. 9, the top (Lo =
0.5 mils) and bottom (Hi = 15.0 mils) Range values are displayed
as two horizontal lines at the top and bottom of the graphics
area. Lo (the minimum limit) is at the top. Hi (the
maximum limit) is at the bottom. 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 range
values, a Cursor can be positioned anywhere between the two
range values to investigate other peaks.
A cursor is used when there are
more than 3 layers. In this example,
the instrument combines
the bottom two layers into a 3.9 mil result. The cursor determines
the top layer to be 5.9 mils. The third layer is therefore
2.5 mils (5.9 – 3.4 (The total of Layer 1 and Layer 2).
Conventional magnetic and eddy-current
gages only work on metals. So the plastics industry has
relied on other measuring techniques 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 plastic
part and coating it at the same time).
These techniques 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.
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.11). 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 coaters
have switched to non-destructive inspection.
Background on Plastic Coatings
What is the Application?
The coating of plastics, particularly
in the automotive industry involves the application of
several coating layers to attain full aesthetic appearance
and protective properties. Not
only does exterior finish tend to be a strong reflection
of the quality and durability of costly consumer products,
but also coatings for plastic components need to address
the challenges unique to plastic substrates including adhesion,
flexibility, and temperature constraints.
Common plastic substrates (i.e. polyethylene, polypropylene,
thermoplastic polyolefin, ABS, nylon, PVC) are nonporous,
resistant to most solvents, and have low surface energy compared
to other materials. This makes the plastic surface
difficult to wet and provides the coatings little opportunity
to adhere by penetrating or physically locking into surface
irregularities. To counter these difficulties, adhesion
promoters are used as paint additives or as primers to promote
adhesion of coatings to their substrates. An adhesion promoter
usually has an affinity for the substrate and the applied
coating, enabling applied coatings to meet the intended performance
In automotive coatings, the term adhesion promoter refers
to the primer (typically chlorinated polyolefin or other
modified polyolefin's), which facilitates adhesion of the
subsequent paint layer to the plastic. Typically adhesion
promoters are applied to achieve a dry film thickness of
0.3 – 0.5 mils (7.5 – 12.5µm). As
the adhesion promoter thickness is below the recommended
1 mil (25 µm) individual layer thickness it may be
difficult for an ultrasonic gage to distinguish it from subsequent
Primers fill any small imperfections from the molding
process and may provide a conductive layer that facilitates
the electrostatic application of subsequent coating layers.
Primer also protects the substrate from potentially damaging
UV energy from the sun, as well as providing resistance to
chemicals (gasoline) and humidity. Often, primers are formulated
to be color keyed to allow minimum basecoat film thickness
and to minimize the effect of stone chipping.
Basecoat is the coating layer
that provides most of the color, physical performance and
aesthetic effects. Fade
resistant basecoats often include special appearance pigmentation
such as the metallic finish common in automotive coatings. Basecoats
can be applied as a single or in multiple layers. Depending
on the application method, multiple basecoat layers can be
challenging for an ultrasonic gage to distinguish between.
Resistant clearcoats form the protective interfaces
from environmental factors such as etch, bird droppings,
car wash scratches and stone chips. Though clearcoats
are used in combination with the basecoat to form the final
finish, acoustically they provide a significant interface
between coating layers and are thus distinguishable from
previously applied layers.
The PosiTector 200 B3 can calculate the average and standard
of a series of measurements for each layer in a
Since automotive coating is one of the most expensive processes
in automobile assembly, manufacturers and assemblers are
constantly looking for technology improvements. Once
such application method is referred to as wet-on-wet where
waterborne coatings are applied directly over top of each
other without allowing previous layers to cure. Such
methods minimize the use of energy, paint, and retooling
requirements, without sacrificing the quality of the finished
appearance. Unfortunately a wet-on-wet coating application
tends to cause a "transition layer" effect (blending
of individual layers). The lack of clear acoustic boundaries
minimizes the capability of an ultrasonic instrument to detect
individual layer thickness.
Why measure with
Manufacturers and applicators alike have long believed that
there is no simple and reliable means for non-destructively
measuring coatings on plastic substrates. Their common
solution was to place metal (steel or aluminum) coupons next
to the part and then measure the thickness applied to the
coupon with either a mechanical or electronic (magnetic or
eddy current) gage. 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
plastic 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
Where is the market?
Over the last several years, the use of plastics has expanded
rapidly. While the automotive industry has certainly led
the way, other industries also make extensive use of plastics. According
to the Society of the Plastics Industry, miscellaneous plastics
products (which accounts for most of the plastics processing
industry), is the fourth largest manufacturing industry in
the United States. Only motor vehicles and equipment,
petroleum refining and electronic components and accessories
exceed it. Though plastics are often colored directly
as part of the manufacturing process, many plastic parts
must be painted to improve appearance, produce a color match
with other parts, improve the stability of the plastic surface,
or produce a desired special effect.
The global market for automotive
paint was $6.6 billion in 2001, according to consultant PG
Phillips & Associates. An
increasing portion of this market involves the coating of
plastics used for bumpers, exterior panels and decorative
trim. Coating applicators and assemblers in the
competitive automotive industry need to meet critical aesthetic
and life expectancy criteria. Since painting is the
most costly process in automotive manufacturing there is
a conflicting priority to minimize the amount of time, materials
and rework involved while still meeting the requirements
of performance-enhancing technology and environmental compliance. Thus
an efficient measurement method is needed to accurately and
reliably detect and correct application problems as early
as possible in the coating process.
Couplant is used to propagate an ultrasonic vibration from
the probe 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.