Measuring Paint 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 paint applied to gypsum board (drywall
/ sheet rock / wallboard).
PosiTector 200 B1 measuring the combined thickness
a single layer of paint and the under layer of primer.
Drywall is typically painted with 3 layers (one primer and
two paint). Traditionally a destructive
test method is used to determine paint thickness. Today,
the primary purpose of ultrasonic testing is to non-destructively
measure the TOTAL thickness of the paint system, typically
in the range of 3 to 5 mils (75 –125 μm). Other
challenges include a tendency for primer to absorb into the
paper membrane of drywall, the effects of paint surface roughness
or texturing, the impact of measuring over joint compound,
and the potential need to measure individual paint or primer
models are ideal for drywall.
- The PosiTector 200 B1 (Standard model) is
the economical and most common solution for measuring the
TOTAL coating system thickness.
- 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
- Using the basic PosiTector
200 B1 to measure total thickness of the paint system
- Measuring on a textured
- The PosiTector 200 B3 graphics
#1: Measuring total thickness
For those familiar with magnetic coating thickness gages, using
ultrasonic coating thickness gages is easy and intuitive.
The measurement method is simple
and non-destructive. The displayed result is a total thickness
of the coating system (primer + paint layers).
The PosiTector 200 B1 is ready to measure most drywall
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 the total thickness of the paint system. This
basic version of the instrument requires no calibration adjustment
for most applications, is mils/microns switchable and has
a large, thick impact resistant Lexan display.
Drywall presents two distinctly different
substrate surfaces that are coated: the face paper
of the wallboard over the untreated area of the wallboard,
and the taping compound over the seams, corners, and fasteners,
(screws or nails). The PosiTector 200 B1 measures both
without any special adjustments.
Both PosiTector 200 models have large LCDs
made of thick,
impact resistant Lexan.
Some walls have coating systems that have been applied over
many years in a number of layers. Our PosiTector 200 B1
is the ideal solution when applicators only need to know
the final, total thickness of the coating system. Since the
primer coat is thin and mostly absorbed into the substrate
material, it has minimal impact on the measured total thickness.
Measuring on a textured
Some painted wall surfaces have a slight surface texture
resulting from the application roller (see Fig.3).
Measuring on a textured surface.
On textured or 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.4. Couplant fills the voids between the probe
and the coating to assist the ultrasonic pulse enter the
Couplant fills the voids between the
probe and the coating.
Sometimes this surface 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 (see Fig.5) to
ignore roughness echoes.
SET RANGE are used to narrow the range of thickness that the
Lo sets the minimum thickness limit and Hi sets the maximum.
Within that range, the measured thickness is 3.3 mils.
The more advanced PosiTector 200 B3 model provides additional
information on surface texturing as described below.
#3: Using the PosiTector 200 B3 graphics capability
The advanced model, called 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.
The gage’s large LCD display is capable of showing
both numerical and graphical representations of the measurement. The
graphical display can be set to appear on the right hand
side of the screen. It shows a graphical
representation of the ultrasonic pulse as it passes through
the coating system.
Some painted wall surfaces have a slight surface texture
resulting from the application roller (see Fig.3).
The B3 model with the graphical display turned on.
In the Screen Capture (Fig.6) the graphical display clearly
identifies the total paint thickness by showing the strongest
return echo from the ultrasonic pulse. The instrument’s
graphical display may provide additional information. In
this example it indicates the amount of surface texturing.
When taking total thickness measurements, periodic high
readings will be displayed when the gage encounters joint
compound covering the drywall seams. The resulting measurement
would include the thickness of the joint compound in its
total thickness calculation. This is due to a greater density
differential between the drywall and joint compound, as compared
to the joint compound and primer. By changing to a 2-layer
application using the gage menu, the gage will individually
identify the total paint thickness and the joint compound
thickness as shown in Fig.7.
The multi-layer measurement capability of the PosiTector
200 B3 also has the potential to identify individual paint
layer thickness, however this would be application specific
since the gauge is limited by the differences in acoustic
velocity between the primer and paint layers. As a
minimum, layers could be individually measured as each paint
layer is applied, allowing the user to calculate the thickness
of the most recently applied layer.
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.
Taking a measurement.
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.8). 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
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
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 betweentrian these layers.
The peaks that the Gage selected are indicated by red triangle arrows (see Fig.10).
In Fig. 10, the top (Lo = 1.0 mils) and bottom (Hi = 15.8 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 top two layers
into a 2.2 mil result.
The cursor determines the top layer to be 1.1 mils.
The second layer is therefore 1.1 mils (2.2 - 1.1).
Conventional magnetic and eddy-current gages only work on
metals. Measuring on drywall required other measuring techniques
- 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
on the wall 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.
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.12). 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
Figure to the right shows an optically magnified
cross-sectional image of the paper membrane and painted
surface. Figure above shows a reticule superimposed
onto image. Units shown are in thousandths of an
inch. Note that the natural paint thickness variation
is between 3 and 5 mils (75 to 125 μm).
With the arrival of ultrasonic instruments,
many coaters have switched to non-destructive inspection.
Background on Drywall Coatings
Gypsum "boards" are formed
by sandwiching a core of wet plaster between two sheets
of heavy paper. When the core sets and is dried, the sandwich
becomes a strong, rigid, fire-resistant building material.
Fire-resistant because in its natural state, gypsum contains
water, and when exposed to heat or flame, this water is
released as steam, retarding heat transfer. Manufactured
in large quantities on continuous machines, gypsum wallboard
and lath, prefinished wallboard, and gypsum sheathing for
use under exterior finishes are among the most important
materials used in housing. ASTM C1597M-04 and C1396C/1396M-13 Standards
describes specifications for gypsum wallboard.
Most drywall primers are water-based, polyvinyl acetate
(PVA) formulations. They are relatively inexpensive and will
not lift the paper of the drywall. There purpose is to seal
the surface of the drywall and joint compound. This helps
ensure that the finish coat will have a uniform appearance.
Why measure with
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
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 included USB Cable and one of the PosiSoft Solutions.