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
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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
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Here is how to properly compare panel readings on a car:
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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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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 (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. 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 will blotch together in sections and form orange peel, leading to higher variations in thickness compared to horizontal panels. 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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thickness varies by color
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 yearsIn 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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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.
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.
Additional Article Sources:
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:
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
…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.