Coating Thickness Measurement
© 2017 DeFelsko Corporation
Automotive Paint Inspection
Updated May 2017
buying decisions at auto auctions require an accurate
assessment of paintwork quality to determine a vehicle’s
DeFelsko manufactures hand-held, non-destructive
coating thickness gages that are ideal for use by body shops,
dealerships, painters, appraisers, inspectors, detailers,
and professional car buyers at auctions. They enable
a quick assessment of the quality of the paint finish
and to verify that the condition of a vehicle matches
its reported history, that is, determine if the vehicle
has been in an accident or experienced other types of
year, millions of used cars are sold at various auction
locations throughout North America. An individual auction
location will often sell more than a thousand cars a
day. Buyers have limited time to determine true value
prior to the vehicle coming onto the block. In addition
to numerous other inspection points, they need to assess
the quality of the paint finish and verify that the condition
of the vehicle matches its reported history. Critically
they must determine if the vehicle has been in an accident
or experienced other types of paint damage.
buyers and inspectors relied only on visual inspections
such as checking body panel alignment and looking for
gaps that might indicate bodywork or panel replacement.
They looked for signs of repainting such as paint overspray
on seals and body openings as well as differences in
paint color and finish throughout the vehicle.
Subtle changes in color, texture or gloss often go undetected
unless the buyer invests significant time to view the
vehicle at different angles and under different lighting
conditions. Visual inspection techniques are particularly
limiting in dimly lit areas, in bad weather (rain, sleet
or snow), or on dirty vehicles.
recent years, buyers have begun to rely increasingly
upon electronic paint thickness meters to accurately
assess paintwork quality. Unlike visual inspections,
these instruments provide reliable and quantifiable measurement
At first, efficient and affordable paint inspection
methods were limited to using magnets or inaccurate magnetic
pull-off testers. Besides having limitations in accuracy
and resolution, the largest drawback of many such magnetic
instruments is their inability to determine paint thickness
on anything other than steel. Modern automotive construction
methods now include materials such as aluminum, fiberglass
Traditionally, steel was used exclusively to manufacture
automobile exteriors because it balanced cost with strength
and machinability. Now aluminum is used for some components
as manufacturers look for ways to reduce weight without
sacrificing safety. Both these metals require painting
for corrosion protection and cosmetic appeal.
Bumpers and fascia systems are commonly made of plastic
and composite materials. Lightweight, they allow designers
the freedom to create innovative concepts. While metal
panels are easily dented by minor impacts, plastic body
panels are more resistant to damage.
On some recently manufactured
cars and trucks it is not uncommon to find all three
materials – doors
and fenders made of steel, roofs and hoods made of aluminum,
and bumpers and mirrors made of plastic.
Improved collision repair quality has made it harder
to spot value-decreasing damage. As large numbers of
low-mileage vehicles come off lease and are being cycled
through commercial and government fleets, inspectors
and buyers rely on recent developments in thickness measurement
technology to help determine vehicle value.
Until recently, consumers requiring an efficient and
affordable means to inspect automotive paint thickness
have mainly been limited to measurement solutions such
as magnets, pen pull-off testers, and a few basic electronic
instruments. Unfortunately most of these solutions have
extreme limitations in accuracy, resolution and in some
instances provide no quantitative results (i.e. only
red, yellow or green lights).
The largest drawback of many such magnetic instruments
is their inability to detect paint thickness on aluminum.
Even on steel some magnetic instruments can be easily
fooled by the addition of steel particles to the filler.
When looking for minor paint thickness inconsistencies
between panels (representing potential repainting), information
provided by an instrument with only a 0.5 mil (13 um)
resolution and 1 mil (25 um) accuracy is extremely limited.
manufactures the PosiTest
a hand-held, non-destructive paint meter that is ideal
for measuring the thickness of automotive paint on aluminum
or steel automotive panels. The PosiTest DFT-F Ferrous is
capable of measuring up to 40 mils of paint on steel
panels, where as the PosiTest DFT-C Combo is
available for measuring on both steel and aluminum panels.
With 3% accuracy and a 0.1 mil (2 um) resolution, either
PosiTest DFT meter is capable of meeting the requirements
of most paint measurement applications.
an alternative, the PosiTector
6000 series of meters
provide similar measurement capabilities with higher
accuracy and resolution. Additional features such as
memory and print capability are also valuable to customers
with a desire to log measurement results.
Based on the
intended application, customized meters and probes are
available that are specifically designed for measuring
on steel, aluminum or both substrates.
Three Measurement Principles
Paint thickness over exterior automotive materials is
best measured with hand-held, electronic instruments.
Three types are available and selection depends upon
the type of coating, the material being painted, and
the size and shape of the part. These instruments use
magnetic, eddy current, or ultrasonic measuring techniques.
Since steel is magnetic, paint thickness over steel is
measured with magnetic (ferrous) gagesemploying mechanical or electronic operation.
gages use a permanent magnet, a calibrated spring,
and a graduated scale. By measuring the force required
to pull the magnet off the coated surface, a thickness
measurement can be determined. Low cost magnetic pull-off
gages provide coarse measurements useful for detecting
bondo or other fillers beneath the paint. Their use
by detailers is limited. Accuracy is typically +5%
with prices around $350. Product details are available here.
Electronic magnetic gages
are much more popular in the detailing industry. They
use a constant pressure probe to provide consistent
readings that are not influenced by different operators.
Readings are shown on a liquid crystal display (LCD).
While most have basic operation, some have options
to store measurement results,
perform instant analysis
of readings, and output results to a printer or computer
for further examination. Accuracy is typically between +1
to 3% with prices ranging between U.S.$300 and $1,000.
Eddy Current Principle for Aluminum
Paint thickness over all other
metals such as aluminum is measured using an eddy current
technique. When the instrument’s probe is brought near a metal (conductive)
surface, a coil within the probe generates an alternating
magnetic field that sets up eddy currents on the metal’s
surface. These eddy currents create their own opposing
electromagnetic field that can be sensed by a second,
Eddy current (non-ferrous) coating thickness gages look
and operate like electronic magnetic gages. They also
use a constant pressure probe and display results on
an LCD with options to print stored measurement results.
is relatively uncommon to find instruments that operate
only with the eddy current principle in this industry.
It is more likely to find gages that incorporate BOTH
magnetic and eddy current principles into one unit. Some
simplify the task of measuring by switching automatically
from one principle of operation to the other, depending
upon the substrate.
These “combo” units
are typically priced between $400 and $1,500.
Ultrasonic Principle for Plastic
An ultrasonic technique is used when measuring the thickness
of paint over non-metal substrates such as plastic and
fiberglass. The probe of the instrument contains an ultrasonic
transducer that sends a pulse through the coating. The
pulse reflects back from the substrate to the transducer
and is converted into a high frequency electrical signal
that is analyzed to determine coating thickness. In some
circumstances, individual layers in a
can be measured. Prices range between $1,800 and
$4,000. For more information on this type
of measurement, click
A complete discussion on these and other types of coating
thickness testing devices can be found here.
Effective Use of a Paint Thickness Meter
There are two basic considerations when measuring a
vehicle with a paint thickness meter.
first consideration is to look for variations in paint
thickness. Even though the expected paint thickness may
not be known for every vehicle type, vehicles should
typically have the same thickness of paint throughout.
Variations in thickness, even subtle changes, can be
indications of paint blending. Paint blending may be
used to hide repairs ranging from minor paint touch ups
for scratching to major body work such as clipping (welding
two half vehicles together). The buyer should thus
be looking for consistent paint thickness around the
vehicle, particularly within a panel. It is important
that measurements be taken near all seams approximately
one inch from any edge. A significant difference in thickness
between adjacent panels is a strong indication that repainting
second consideration is to investigate prime spots for
collision or rust damage. Vehicles from northern climates
tend to rust near the bottom, particularly lower door
edges and panels subject to splash from the tires. Vehicles
from coastal climates that receive ocean spray are more
likely to rust around the hood, trunk and upper areas
of fenders and doors, particularly near molding, handles
and emblems. The middle area of fenders, rear quarter
panels and doors are common areas for damage due to bumps
readings in these areas are thicker than over the rest
of the car, additional measurements should be taken in
surrounding areas to verify the results and to determine
the reason for repainting. One of the primary advantages
of using a paint thickness meter is the speed at which
multiple readings can be taken.
An example of how repaired rust damage can be detected
using a paint meter, read the Case
A paint thickness instrument is a powerful tool for
finding evidence of accidents, patching of rust spots,
and past paint repairs to delaminated surfaces. Problems
under the paint will eventually develop with time. No
matter how good the paint job, poorly prepared or damaged
bodies will shorten the life of the vehicle. Thus a vehicle
that has been repainted, regardless of the cause, can
have a reduced resale value. The resale value decreases
more significantly with high value (hi-line) vehicles
and newer models.
Proper inspection can prevent unplanned future repainting
costs. High quality repainting costs can be as high as
$3,000 and increase significantly when bodywork is required
to repair hidden collision or rust damage.
In addition, inspectors stake
their reputation, future business and possible lawsuits
on their judgments. If they fail to identify a vehicle
that has sustained unreported damage it can have a
significant financial impact. The
relative low cost of paint thickness meter greatly reduces
this risk in addition to paying for itself due to increases
in efficiency while inspecting.
Reports are detailed legal documents that contain an
inspector's assessment of any damages to the vehicle
along with estimated repair costs and digital images
if requested. The primary purpose of a Condition Report
is to accurately communicate the condition of the vehicle
to the consignor. The report is a full evaluation
of the vehicle's condition, and contains recommendations
from the Condition Report Writers on Recon work that
can or should be done to increase the salability of the
On-line sales further increase the need for pre or post
sale detailed Condition Reports, as the consumer does
not have the opportunity to verify the condition of the
vehicle in person. Types of on-line sales now include:
auction simulcasts where customers bid on-line against
others physically attending an auction; Cyber (Smart)
auctions where all bids are made on-line); Cyber lots
(virtual automobile lots) where customers can browse
and make purchases anytime); and dealer exchange sites
that facilitates the buying and selling of wholesale
vehicles (functions as an eBay for automobile dealers).
Who Needs to Measure?
The primary market for paint thickness meters is the
$380 billion used vehicle remarketing industry. However,
many of the same measurement principles and benefits
exist in other automotive markets including body shops,
painters, repainters, refinishers and detailers. Anyone
with the need to control the amount of paint being applied
or removed to a vehicle would recognize considerable
benefits from a paint thickness meter.
Large auto auction companies have
dozens of locations and thousands of employees, with
the largest moving up to 9000 vehicles per week from
a single location. As
part of their inspection, certification and reconditioning
programs, auction houses often train and employ their
own Condition Report Writers. Condition Report Writers
may also work for specific dealers or as independents.
Similar to the Condition Report Writers found at auctions,
many other professionals have similar needs for inspecting
paint thickness as part of determining a vehicles value.
For dealerships, the Used Car Manager is typically responsible
for appraising and accessing the amount of damage to
a potential trade-in. Likewise insurance adjusters,
professional car buyers and collectors have similar requirements.
An important market for paint thickness meters is the
used vehicle remarketing industry. Many other professionals
have similar needs for inspecting paint thickness when
determining vehicle value. For dealerships, the used
car manager is typically responsible for appraising and
accessing the amount of damage to a potential trade-in.
Likewise insurance adjusters, professional car buyers
and collectors have similar requirements. Other automotive
markets that benefit from these instruments include body
shops, painters, repainters, refinishers and detailers.
Anyone with the need to control the amount of paint being
applied to or removed from a vehicle would recognize
considerable payback from a paint thickness meter. Affordable,
reliable, and simple to operate, they have become an
accepted and reliable tool for car buyers and inspectors
to assess the value of automobiles.
The related article “Automotive
Paint Detailing” contains
additional information, including:
- A detailed discussion of DeFelsko’s
3 options to the auto industry.
- How to take measurements
with a paint meter
- Answers to a series of popular
questions including “What
is a mil?” “What should the paint thickness
be?”, “What does F and N mean on the display?”,
and “Do I have to re-calibrate my meter often?”
The vehicle shown below had undergone significant bodywork
on its right front panel. Even when notified that damage
existed, the damage was not visually detectable by five
trainees. The five trainees were then tasked with finding
the damage with a paint thickness meter. By quickly taking
measurements next to the various seams in the vehicle,
trainees typically took less than two minutes to locate
the damaged panel while pronouncing the remainder of
the vehicle fit.
Measurements taken around the
vehicle ranged between 4.2 and 4.5 mils (105 – 115
um). Figures A and B depict seam measurements taken
with a PosiTest DFT Combo meter. The readings
below are within the expected range and
were close to readings taken across the remainder of
Door panel measurement to the left of the seam.
Door panel measurement to the right of the seam.
But measurements taken closer to the bottom of the panel
(Figure C) had a significantly higher reading of 10.6
mils (270 um). Moving further down the panel (Figure
D) readings increased to 36.8 mils (930 um) indicating significant
Higher readings indicate paintwork.
Significantly high readings indicate bodywork.
A review with the vehicle owner verified that bodywork
had been completed to repair rust damage behind the front
right tire, not unexpected for an older vehicle that
has been driven all year round in Upstate New York.