what a paint thickness gauge REALLY tells you

understanding the readings!

Author: Andre Clemente, Founder of New Old Cars, LLC ©
About the author
Last updated: July 16, 2020

Prerequisite: A paint thickness gauge (aka paint depth gauge) is used to measure the thickness of a vehicle’s paint job to determine whether or not a car has received paintwork after it left the factory (either due to a prior accident, paint damage, etc). A repainted panel will generate a higher reading than a panel wearing original paint. If you are new to collecting and wondering: why do I need to know this? Have a look at our True Value of Original Paint article.

You just spent hundreds of dollars on a fancy new paint thickness gauge, determined to arm yourself with the ability to differentiate original paint from a respray on any car. Excited, you go around your first subject and find that, no matter where you stick the gauge, you always get a different reading – even within the same panel. The readings aren’t as high or as low as you were expecting, and deep down inside, you can’t confidently explain if it’s original or not. What gives?

Ask yourself – can you really explain what you’re looking at? A lot of people can’t. And for good reason – authenticating original paint with a gauge isn’t as clear-cut as the internet leads you to believe.

NOC has put together an easy to read article explaining that, while it’s not an exact science, the readings become easier to comprehend once you accept the realities outlined in this guide. You will learn why:

G series Porsche 911’s (1976-1989). I entered the business right when air cooled Porsches began to skyrocket in value. Strong demand in Europe, combined with the strength of the Euro over the dollar meant European dealers were swooping them up in the states and shipping them overseas for resale – at an alarming rate. The cars had to be as original as possible, including the paint. It was during this time period when I witnessed the “rise of the paint thickness gauge”. Everyone in the business had to have one, even if they weren’t necessary. Quality paint thickness gauges have been around for years and were reasonably priced by this time, further aiding in their growing popularity.
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  • There isn’t a “factory” thickness rating for your car – factory readings can vary drastically between panels (and even within the same panel)
  • There’s no “universal” number to adhere to when comparing readings
  • If you’re looking for “anything higher” than a particular number, you’re doing it wrong – especially if you think it applies to any car
  • Why some panels on older cars show a substantially higher reading than the rest of the car
  • Different colors and surface shapes generate different readings
  • Why readings can vary drastically between different model years and manufacturers
  • A resprayed panel may not read as high as you think
  • Why you should never rely on a paint thickness gauge alone

Most importantly, NOC explains what you should be looking for when conducting readings on a vehicle.

While these gauges can speed up the job, they can also add to the confusion and lead to inaccurate conclusions. However, this article will make everything easier to understand so you have more confidence out in the field when buying your dream car. Combined with our article on verifying original paint, you will have all the confidence you need to differentiate original paint from a respray.
 

what to look for

Let’s start with the basics. You must understand cars are painted in a homogeneous manner at the factory. Since the complete body shell is e-dipped and electro-coated, factory paint thickness is quite uniform all the way around. The high tech paint applicators combined with intense quality control mean that there will be little variation in paint thickness between panels. So, assuming the entire car hasn’t been repainted, you are looking for uniformity between all of the metal panels on the car. If the driver’s door reads 110 microns and the adjoining fender reads 220 microns, you can absolutely bet that fender has seen paintwork. A repainted panel will always read higher than the original paint, but when you’re out in the field, you’ll come across a dizzying array of numbers. Exactly how much higher will it read? How do you truly differentiate a factory paint reading from a respray?
The individual layers that make up the paint on a modern car, provided by BASF Coatings, one of the major suppliers of paint for automakers.
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Mils or Microns? Paint thickness can be referred to in either mils or microns. One mil is equal to one thousandth of an inch, while one micron (µm) is equal to one millionth of a meter. One mil is equal to 25.4 µm (microns). While paint depth gauges can display both, most factory paint departments prefer microns.

 
The reality is that the thickness of a repainted panel can vary astronomically. There is no “magic number” where a factory paint reading stops and a respray starts, as all manufacturers are different. A few people I met in the business insisted things like “any reading over 200 microns is a respray”. While that statement may ring true most of the time, it doesn’t acknowledge the fact that a repainted panel can read substantially lower than 200 microns. Have a look at the fender on this 2015 VW GTI, which received paintwork from a minor swipe. This fender shows readings as low as 148 microns – a measurement that is on par with many factory readings. However, the neighboring panel, wearing its original paint, reads 102 microns. In comparison, the opposite side of the car varied only by +/- 15 microns between the same locations.
 
This fender on my fathers car has been repainted. Many people will look at this reading alone and assume it is original paint. They would be incorrect – the neighboring door had consistent readings of ~102 microns. There should never be a difference of nearly 50 microns when comparing these two surfaces on a 2015 car. In comparison, the opposite side varied only by +/- 15 microns between the same locations.
On a side note, you have to admire how Volkswagen’s paint process can provide a finish with this much gloss and depth at only 102 microns thick. This car has never been wetsanded or detailed with a machine – my father bought the car brand new.
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A great example of how a repainted panel holds up to damage. Our GTI was bumped again, damaging the repainted fender. Look closely at how the primer layer separated upon impact, exposing the sheetmetal underneath. NOC has established that factory paint and primer do not chip like this – only a respray. The factory primer layer is far too strong to tear apart in this manner (you can tell it was a light impact because no dent was formed. The sheetmetal still retains its shape).
Zooming in. This what 150 microns of paint thickness looks like. Notice how the primer/e-coat layer is lifting at the edge of the chip. Not factory! You couldn’t replicate a paint chip like this in a million years on factory paint.
 

Here is how to properly compare panel readings on a car:

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Make it a habit to wipe the surface clean (you don’t want the tip to measure through dirt particles, screwing up the readings). I start with a measurement close to the edge of the panel. Now move to the adjoining panel and take a measurement next to the same edge.
You are successfully comparing 2 measurements of similar surfaces on adjacent panels. These measurements shouldn’t be very different.
 
In reality, the majority of the repainted panels I’ve measured will read 200 microns or higher. Some can be as ridiculous as 350 or 400 (the sky is the limit), and some can read as low as the GTI at 148 microns. Having conducted thousands of readings, I can tell you from experience that a difference of 40 microns or higher between adjacent panels (measured on comparable surfaces) is a red flag, especially on modern cars. It will trigger me to inspect deeper for evidence of paintwork (where I’ll usually find some. There are many variables effecting paint thickness, and for that reason I never rely on a gauge reading alone. Final verification is always done with a visual inspection for evidence of paintwork.)
 
 
And wait a minute, if a car is painted so uniformly at the factory, then why would there be any variance? (sometimes as high as 35 μm or more)?
 
 

the reading varies depending on where you take the measurement

There are multiple explanations for this.

When painting a car, the hard to reach areas (like the door jambs, engine bay, and trunk rain gutters) need to get painted first. This is one of the only paint procedures completed by hand (although manufacturers are slowly switching to 6 axis robotic sprayers for this job). Regardless, the overspray from this procedure will unavoidably find its way onto the main exterior surfaces (quarter panels, fenders, C pillars, etc). This overspray adds to the overall thickness of the finished paint job, which is why you usually get slightly higher readings in these areas:

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A Quattroporte at the Maserati factory receiving paint in the engine bay, rain gutters, and underside of hood. For this job, hand application is still the best option for certain carmakers. Source: How It’s Made: Maserati Quattroporte (via the MotorTrend channel)
When ensuring complete coverage of the hard to reach places, overspray will result around the edges. The overspray will add to the overall paint thickness in the affected areas once the paint job is completed. Hand application will generate more overspray than robotic 6-axis sprayers.
 
In addition, while the factory paint applicators (known as electromagnetic bell cup atomizers) are the most advanced in the industry, they aren’t perfect. There are multiple sprayers painting a car at one time, so body panels can receive erratic, overlapping coverage from the neighboring sprayers (overspray). Even though the charged body attracts the oppositely charged paint from the sprayer, it doesn’t guarantee the paint will coat the entire panel in a perfect uniform manner (although it’s pretty close). And while the sprayers can achieve an incredible 90% transfer rate (the % of paint leaving the sprayer that actually hits the panel), there’s still 10% that could theoretically land anywhere on the car as it settles. This will easily contribute to erratic paint depth readings throughout the body, but with intense quality control, we’re only talking a matter of fewer than 35 microns at most on modern cars.

Also, modern car design consists of vastly different shapes and curves throughout the body (and even within the same panel). Paint cures differently based on the shape of the panel due to gravity, which will contribute to mismatched readings between dissimilar surfaces.

During baking, paint flows and settles as part of the curing process (thanks to gravity). Paint that’s curing on a flat, horizontal surface doesn’t have to defy gravity like paint on a vertical surface. The reduced flow control on the sides of the car means the paint can blotch together in sections and form orange peel, leading to the variations in thickness. This is one of the many reasons why you see pronounced orange peel along the sides of the car more so than the hood, roof, or trunk lid. Bottom line: expect to see different readings on dissimilar surfaces of the car.

 

cars that see powder coating are more uniform and have a higher film thickness

A car’s paint job is generally made up of 5 layers: Phosphate, E-coat, primer, basecoat, and clearcoat. Certain manufacturers apply the primer layer via powder coating, which can easily be twice as thick as your standard waterborne primer layer (60 µm compared to ~30µm for waterborne primers). The result is a better film build and greater stone chip resistance. Ferrari, one of the most famous manufacturers to utilize this tech, builds cars with some of the highest paint film thickness in the industry. Ever since the Italian marque began using powdered primers in 2004, typical paint thickness ranged from 150 to 200 microns on almost all of their cars. The California T in Rosso Corsa, for example, has a minimum thickness of 200 microns once cured. (Models like the Ferrari F40, which was known for having ultra thin paint, are an exception – it was the result of weight saving tactics in the late 80’s).

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A 599 receiving Ferrari’s advanced powder coat primer at the Maranello factory. Ferrari is the pioneer in powdered primer technology.
A factory paint specialist verifying the 599 has a paint thickness of around 165 microns. Since the bodywork on most of their models are aluminum, Ferrari has to measure their paint ultrasonically.
 
Liquid paints contain solvents, which provide flow control and allow the “hills” and “valleys” in the paint to level out. Solvent also helps hold the paint together as it evaporates. Powdered paint, on the other hand, doesn’t have solvent (since powder is a solid and not a liquid, it doesn’t naturally flow, so no solvent is used). Instead, extremely hot baking temperatures have to melt and “fuse” the powder to the body. So while waterborne paint sees all of its liquid (the water) evaporate, powder coating experiences no loss in material through evaporation. This one of contributors towards a higher film thickness.

The chemistry of powder coat leads to another advantage:

“The ability of powder to apply with no sag and run behavior results in a film build varying only within a few microns between horizontal and vertical [panels]. This attribute produces a homogeneous and consistent appearance over the entire body.- PCI Mag

Since powder is immune to the flow control problems experienced with waterborne, the consistency in paint thickness between panels is respectably high, and the consistency between individual cars is just as incredible. According to an “Ultimate Factories” documentary of Ferrari’s paint shop, a Rosso Corsa 599 will vary a maximum of one micron between another Rosso Corsa 599 when measured on the exact same part of the body.

Note how much thicker the film build of powdered primer is over the other, more traditional coats.
Photo Credit: Automotive Paints and Coatings (2nd Ed)
 
 

thickness varies by color

 

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The manufacturer ensures that each paint color is applied in the correct minimum thickness. It’s such an important factor that most manufacturers take a “touchless” electronic measurement on every car immediately after the paint cures, with the objective of 100% quality monitoring.
Yes, the factory has a different film thickness requirement for each color. Certain paints require heavier coats for more coverage, as not all colors have the same “hiding power” (opacity) as other colors. According to Automotive Paint and Coatings, 2nd Edition:

“Base coat film thickness depends on the hiding power of the base coat, which is again dependent on the pigmentation. For silver shades, which exhibit good black–white hiding, typical film build is around 10 µm, for white shades around 20 µm, and for yellow/red shades up to 30 µm.”

Considering 25.4 microns equals 1 mil, that’s almost a 1 mil difference between two colors! Remember, the manufacturer wants to use the least amount of paint possible to reduce cost, weight, and emissions. If they can achieve similar gloss with less paint, then they will do so.

Keep in mind that premium or special edition colors usually register a higher thickness (such as tri-coats, etc). The lowest paint thickness you will come across is usually the single-stage white paint used on commercial trucks, which can frequently register less than 95 microns of overall paint thickness.

 
 

factory paint chip protection can skew results

On many cars built in the late 80’s and throughout the 90’s (some early 2000’s as well) you will notice the front portion of the hood and fenders will read substantially higher than the rest of the panel (variation can be well over 50 microns on factory paint). This is due to a thicker application of primer, as these specific areas are highly prone to stone chips (which can expose the metal underneath and lead to rust). The manufacturers had to address this potential rust problem if they wanted the body to last the length of the warranty, and a common solution back then was to apply a thick build of anti-chip primer on the most susceptible areas.

However, modern paint application and curing tactics have become so advanced that “additional applications of stone chip primers, as used in the 1980s and 1990s on front ends, are not necessary any more” – Automotive Paints and Coatings, 2nd Ed.).

 
 
Illustration A shows the area in which anti–chip primer is used. This material improves stone chip resistance in critical areas such as the leading edge of the engine hood and front fenders. This photo was pulled straight from Toyota’s Collision Repair  Bulletin #52, dated February 1994.
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Instead, today’s cars use a urethane and/or PVC coating along the underbody, wheel wells, and rocker panels. The PVC coating is applied prior to paint and will appear to have a rough, coarse texture – it is phenomenal at providing stone chip resistance to body panels, especially the rockers. These layers can add 50-60 microns or higher to the paint thickness.

Since it isn’t clear exactly when the practice of front end stone chip primers started or ended (and when the transition to modern PVC occurred), I personally disregard all of these areas when taking measurements. You’ll just confuse yourself with what’s original and what isn’t, and quite frankly, there are other ways to spot a repaint .

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Urethane and/or PVC was used on the highlighted areas to increase stone chip resistance. This is still the case on modern cars (PVC), but the thickness is much lower due to the strength of modern paint.

Touch up those rock chips! There is a reason manufacturers go through the effort of supplying paint touch up sticks at the dealer. It’s not just for cosmetic touch up – it is intended to cover up exposed metal and prevent rust.

 

evolution of paint thickness through the years

Back in the day, manufacturers relied on higher paint thickness to address the following problems:

  • Cover up uneven surface roughness of the metal body stampings
  • Create a stronger defense against stone chips and paint damage
  • Promote even, uniform paint coverage throughout the car

In addition, cars from 1984 to ~1992 were built when e-coating thickness was at an all time high (35 μm – see chart), further increasing the overall paint depth.

Starting in the early 2000’s, many manufacturers with plants in North America were experimenting with powder coated primer application, which added to the overall thickness.

One could argue that the above two time periods was when factory paint thickness was at its absolute highest, but remember – depending on how many details and car washes a vehicle from that generation has seen, those readings may not be as high when measured today.

Despite lower thickness, the corrosion protection and durability of today’s paints is about two times higher than what it was 25 years ago. Less paint means reduced weight and cost
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In fact, from personal experience, I find that it wasn’t until the ~2007 model year when most mainstream cars were consistently in the 100-140 micron range (this translates to about 9–16 Kg of paint per car). By this time, all production paint shops have been comfortably upgraded to waterborne, and technology has advanced enough at this point to address the above bullet points and reduce paint thickness to what it is today.

With a few exceptions, modern paint finishes will rarely ever exceed 190 microns, whether powdered or waterborne. The majority remain in the 100-140 micron range.

The thickness has been reduced as clear coats have become harder, stronger, and more UV resistant, therefore requiring less film build for the required protection. Colored primers and tinted clearcoats help produce better coverage with less paint. Also, modern robot application can apply a single coat so uniformly that coating thickness can be further reduced while maintaining the same coverage as paint jobs of the past. The cost of owning such equipment has gone down as well, allowing virtually all automakers to benefit from this technology.

Q: How can I tell if my car received powder coating when new?
A: You need to investigate which assembly plant your car was built at. When you find it, go on the assembly plant’s website where you will usually find a timeline of when certain advancements in technology were implemented (a google search will uncover this too). Just because a car company introduces a new paint technology in year X, it doesn’t mean every plant will receive this new tech immediately (it’s usually only implemented at 1-2 plants at first). It takes years for manufacturers to update all of their paint shops to the new tech. For example, Toyota introduced waterborne paint in 1992 at their United Kingdom plant and then on Line No. 2 at their Kentucky plant (TMMK) in the United States in 1993. It wasn’t until 2000 that Toyota commenced the use of waterborne paint at its Takaoka Plant in Japan, and it wasn’t until 2005 when the switch to waterborne was completed at all factories worldwide. There were many 90’s and 2000’s Toyota models (including the MKIV Supra) that were left out and didn’t receive waterborne paint throughout their entire production run.

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BMW’s paint shop at the Dingolfing plant applies 5 coats to the body: phosphate, E-coat, primer, base coat (color), clear coat. About a 3/4 gallon of paint is used for the base coat alone. Despite receiving a unique powdered clear coat, the final finish averages a remarkable 120 microns thick once cured.
According to the paint shop at the Porsche Leipzig Factory (one of the most advanced paint facilities in the world): “… all four coats of paint make a total layer of approx. 0.13 millimeters, which is only slightly thicker than a human hair.” This translates into 130 microns.(Source)
According to Chris Wilkes, head of Paint Planning at Rolls Royce, a typical Rolls can register 150 microns or higher due to the many layers of paint applied by the factory. Note that this reading is taken after numerous hours of wet sanding to remove any trace of orange peel.
(Source)
 

why do repainted panels read higher than factory paint?

A repainted panel will always have a higher paint thickness than factory applied paint. The electromagnetic bell cup sprayers apply charged paint to an oppositely charged car body, resulting in a 90%+ transfer rate (most average body shop HVLP guns have a 40% transfer rate). This high transfer rate means less paint is wasted per coat, and combined with the charged surface attracting paint like a magnet, a single basecoat application is all that is needed to provide even coverage throughout the entire panel, while achieving excellent depth and gloss. In comparison, body shops need 2-3 basecoats to achieve the same coverage and gloss.

The factory paint shops also utilize techniques like tinting the primer and clearcoat to compliment the basecoat color, which further reduces the amount of paint needed to cover the panel. Less paint equals a lower cost for the manufacturer, and the paint department applies such an even coverage that film thickness will register as low as 100 microns while still looking brilliant (yes, brilliant … minus all of the orange peel. This is a topic that has earned its own article).

Now, this isn’t an insult to the refinishing industry – they do a fantastic job considering what they have to work with. They aren’t as worried about ensuring the thinnest coat possible – the end goal is to match the repair quality with the rest of the factory paint job, and this requires several coats. The result is a paint finish that looks equal to or better than the factory paint.

Common Question: Does the temperature of the paint affect my reading?

Answer: Technically, yes, but the difference in paint thickness (modern cars) between hot and cold days is only about two microns. And guess what the accuracy of most paint depth gauges are? Plus or minus 2 to 3% accuracy, which when reading 150 microns, translates to +/- 3 microns! So don’t worry about it!

Common Question: When measuring my car, one or more of the body panels shows a reading indicative of a repaint. However, I bought the car brand new, straight off the lot, and never had any paintwork done. What’s wrong here?

Answer: You might not have had any paintwork done under your ownership, but the dealership or transporter most likely did, before they sold it to you ;) Learn why this problem is more common than you think.

Author:

Andre Clemente, Founder of New Old Cars, LLC ©
Article last updated: July 16, 2020

 

About the Author: Andre Clemente, a member of the Society of Automotive Historians (SAH), has spent over 12 years in the business of buying and selling cars – half of those years were dedicated to the classic car/sports car business. As an automotive paint fanatic, Andre has been hyper-focused on learning paint correction and inspecting automotive paintwork, working alongside veteran dealers, brokers, and a licensed Concours judge in the process. Years of real-world practice and application gave him the experience to identify inaccuracies and myths that are widely accepted when authenticating a vehicle’s paint job. Rather than keep his knowledge as a trade secret, he has decided to share the research and insider details he’s learned to help educate the collector car community, and ultimately shine a brighter light on the cars truly wearing their original paint.

 

The top coating manufacturers in the world (BASF, Axalta, PPG, as well as paint evaluation tool suppliers like BYK) spend hundreds of thousands of dollars conducting tests, case studies, and other forms of research. This material is supplied to carmakers to help educate them on why they need to invest in their tools and equipment, which would help save money in the paint shop and produce a better looking product. Much of this material is available online in the form of textbooks, brochures, in-depth papers, and more. While highly technical, NOC’s silly obsession for knowledge on this topic means we dissect virtually anything we can get our hands on, pick out the interesting stuff, and highlight it in our articles. Below you’ll find some of the material used for this article:

Additional Article Sources:

Streitberger, Hans-Joachim, and Dössel, Karl-Friedrich. Automotive Paint and Coatings, 2nd Edition. WILEY-VCH Verlag GmBH & Co, 2008.

Streitberger, Hans-Joachim, and Goldschmidt, Artur. BASF Handbook on Basics of Coating Technology, 3rd Revised Edition. Vincentz Network, March 2018

Film Formation and Surface Tension Studies Of Powder Coatings

BYK Additives and Measurements: Introduction to Film Thickness

Evolution of the Automotive Body Coating Process: A Review

BASF: Car Finish: Higher Gloss and Fewer Scratches

…and many more! NOC takes incredible pride in posting only the most accurate information with the help of credible sources. Now, because some links are no longer active, not all sources are posted here. These links have been removed from the source’s website for unknown reasons. However, NOC downloads and retains all sources used to stand by every statement in this article. This is done for all articles on our website, and NOC is happy to share this information with the public. Your trust is our number one priority.

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