A paint thickness gauge (PTG, also known as a coating thickness gauge) is an electronic device used in the auto detailing industry to accurately measure the thickness of a car’s paint and/or any protective coatings applied to it.
In this article, I’ll explain what paint thickness gauges are used for, how they work, when to use one and how to choose one from the many options available online.
What Does a Paint Thickness Gauge Do?
A paint thickness gauge measures paint thickness. To be more specific, it measures the amount of “film build” from paint or ceramic coatings on the surface.
When Should You Use a Paint Thickness Gauge?
There are two situations in which you’d want to use a paint thickness gauge when detailing a car:
- Measuring Clear Coat Thickness Before Paint Correction: Clear coats on factory painted cars are incredibly thin. If you polish the paint too aggressively, you may remove too much clear coat and “burn” the paint, that is, create a hole in the clear coat. Using a paint thickness gauge can tell you how much clear coat you have to work with so you have a better idea of how aggressive you can be without damaging the paintwork.
- Measuring Ceramic Coating Thickness After Application: Some ceramic coating manufacturers advertise the thickness of their coating as a feature. Some detailers will use a paint thickness gauge after application to verify the coating thickness and ensure the advertised level of film build was achieved.
How Do Paint Thickness Gauges Work?
Paint thickness gauges for use on automotive paint typically use electromagnetic induction to measure the distance between the paint surface and the body metal underneath.
When you place the gauge sensor on the paint surface, it generates a magnetic field. The sensor then measures how much the magnetic attraction between the body metal and the sensor is distorted by the paint and coating film in between. That reading is then converted into a measure of thickness which appears on the display screen.
From there you can record the measurements on a car body panel diagram like the one below. Some brands also provide a cable or Bluetooth connectivity to record measurements.
How Much Do Paint Thickness Gauges Cost?
Like most things, you get what you pay for. Price will vary depending on features, quality, accuracy and brand name.
 |  |  |
|---|---|---|
| MODEL: QNIX 4500 | MODEL: Rhopoint CTG | MODEL: Abizarch HW300 Pro |
| COST: $1000+ | COST: $170 | COST: $18.99 |
| ACCURACY: +/- 1% | ACCURACY: +/- 2% | ACCURACY: +/- 3% |
| Request Quote | Purchase Link | Purchase Link |
Ultra-high accuracy paint thickness gauges like the QNIX 4500 are typically used in industrial settings where accuracy is crucial for mass production of cars.
Car detailers will usually use mid-range gauges like the sub-$200 Rhopoint Coating Thickness Gauge because they don’t require the same level of accuracy to determine whether paint is safe to do correction on.
I strongly caution against using extremely cheap thickness gauges like the Abizarch HW300PRO. While you may think you’re getting a deal, the low accuracy and poor build quality will greatly affect performance.
For the “how to” section of this article, I’ll be using the Rhopoint Coating Thickness Gauge as this is one of the most common gauges used by detailers and is sold on most car detailing supplies stores.
Full disclosure – my Rhopoint CTG was provided to me at no charge by Darryl Case, president of Rhopoint. I have an excellent relationship with Darryl and he knows I teach a lot of car detailing classes and create a lot of content for the detailing industry so he provided me with some coating thickness gauges for my students to use and get first-hand, real-world experience. He knows that if students get to use the tool and it performs as advertised it will sell itself.
What Paint Thickness Gauge is Best?
The best paint thickness gauge is always the one that is most accurate. When purchasing a paint thickness gauge, look at the specs for accuracy estimates.
Typically manufacturers will list a margin of error for the sensor reading, and the higher the margin of error, the less accurate the gauge will be. Margins of error around +/- 1-2% are what you want to look for. I would avoid anything higher than a 2% margin of error, that’s just not accurate enough for the job.
How Do I Know if My Paint is Too Thin for Paint Correction?
Generally speaking, whether you’re measuring factory or custom paint, thicker is better. The higher the number, the more protection the car will have against the elements. Overall, cars with healthy factory paint should measure 75 microns (3 mils) on the low end and up to 200 microns (8 mils) on the high end.
Years ago I wrote an article on paint thickness gauges where I coined the term “Big Picture Indicator”. PTGs are Big Picture Indicators in the sense that they tell you what the “big picture” of the paint’s thickness is, or whether it’s safe to do aggressive paint correction on.
This is something I call a “Go” or “No Go” decision.
If you’re working with factory paint and the thickness is on the lower end, that’s a NO GO for any heavy paint correction like using aggressive compounds or sanding.
If you’re working on custom or resprayed paint, either as part of collision repair or a custom build, the painter will typically spray a thicker coat than the factory, so your readings will usually be on the high end or higher. That means you are a GO for extremely aggressive paint correction like wet sanding to remove orange peel texture.
If you’re looking for a definitive guide to how thin is too thin…
- TOO THIN: 1-6 Mils (25.4-127 Microns)
- SAFE: 6-100 Mils (152.4-2540 Microns)
How to Use a Paint Thickness Gauge
First let’s look at what comes with the Rhopoint Coating Thickness Gauge. From left to right we have:
- Coating Thickness Gauge
- Owner’s Manual
- Lanyard
- Calibration Plates
- 100um Plastic Shim Film
- USB Cable
- Batteries (not pictured)
Step 1: Install Batteries
The first step to setting up the Rhopoint Coating Thickness Gauge is installing the included batteries into the battery compartment located on the back of the tool. Note that you will have to remove the rubber overmold to access the battery compartment. Replace the rubber overmold when you’re done to protect against scratches if your gauge accidentally falls on the paint.
Step 2: Calibrate Gauge
In order to get accurate paint thickness readings you need to calibrate the gauge. There are two metal calibration plates that come with the gauge — one for ferrous (containing iron) metals and one for non-ferrous metals (aluminum, copper, brass, etc).
This is so the gauge can properly measure paint thickness on the different types of metal used on modern car bodies. Also included is a 100 micron plastic calibration shim for placing on top of the metal plates to test accuracy.
Start by measuring the ferrous metal plate with the Coating Thickness Gauge. The gauge should read “0.0”. Repeat with the non-ferrous metal plate, which should also read “0.0”.
Once you’ve confirmed the Coating Thickness Gauge is showing a 0.0 reading on an uncoated metal surface, place the plastic calibration shim over the ferrous metal plate and measure with the gauge. You should get a reading within +/- 2 microns or between 98 and 100 microns. You can see below that the accuracy is within 3.4 microns, which is still incredibly accurate.
Now do the same with the non-ferrous plate. My reading shows an accuracy within +/-1 microns.
IMPORTANT: In addition to calibrating before first use, the Rhopoint Coating Thickness Gauge should be re-calibrated whenever you change batteries, measuring materials or if there’s a dramatic change in ambient temperature, like going from an air conditioned shop to a hot humid outdoor environment.
To re-calibrate, hold the powered-off gauge above the calibration plate. Press and hold the power button then immediately press the probe against the calibration plate. Hold the power button until the display reads “CAL FINISHED”. The calibration is then complete.
Once you’ve confirmed the gauge is calibrated you’re ready to start measuring. But first, let’s go over the different settings.
Step 3: Adjust Settings
There are a number of different settings you can change to your desired preference via the display. This includes:
- Probe: Auto – Mag – Eddy
- Language: Eng – Ger – Spa – Tur – Rus – Ukr
- Unit Reset: Microns – Mils – Millimeters
- Bluetooth: Disable – Enable
- Switch Font: Default
Probe Setting
There are three settings for the probe:
- AUTO: The probe automatically detects the substrate type and adjusts the mode accordingly.
- MAG: The probe uses magnetic induction to measure paint thickness, which is used only for ferrous metals.
- EDDY: The probe uses eddy current testing to measure paint thickness, which is used only for non-ferrous metals.
I set the probe to “AUTO” to keep things simple.
Language Setting
On the language settings screen there are settings to change the display readout language to English, German, Spanish, Turkish, Russian and Ukrainian.
I set the language to “English”.
Unit Setting
There are three unit settings available on this device. They are as follows:
- MM: Millimeter, one-thousandth of a meter.
- Mil: One-thousandth of an inch.
- μm: Micron, one-millionth of a meter.
I recommend setting your coating thickness gauge to microns as it is the smallest and therefore most accurate unit.
While it’s common to measure car paint in mils in the painting world, there’s just too big a difference in scale between one-thousandth of an inch and one-millionth of a meter to get an accurate reading while doing paint correction.
The more accurately you can measure the total film build, the better understanding you will have of the thickness or thinness of the paint on the car in front of you.
Bluetooth
I set mine to “enabled”.
Switch Font
I believe this setting is for optimizing readability. I found the default font to be perfectly readable, so I kept the default setting on here.
Reset
This clears any past data.
Step 4: Measure Paint
Before you measure the paint on any vehicle, ensure it is clean and dry first. The Coating Thickness Gauge will not be able to get an accurate reading if the paint is dirty.
When you’re ready to measure the paint thickness, take the gauge probe and hold it completely flat to the surface to ensure maximum accuracy.
Repeat this process across all major body panels and record the measurements in a notebook or by taking a picture of the readout with your phone.
What Do the Green and Red Indicator Lights Mean?
When the coating thickness gauge is able to accurately measure the underlying paint, the light on the upper right will flicker green, like seen below.
When the measurement is faulty, the light on the upper left will flicker red as seen below while trying to measure the cardboard box the gauge came in.
What Does “No Metal” Mean?
When the gauge shows a reading of “No Metal” it means the probe is reading a surface with no metal substrate. On a vehicle this typically indicates some type of composite or plastic filler such as Bondo which is often used in the body shop industry for repairs.
What Does “Overflow” Mean?
A reading of “overflow” on your coating thickness gauge is supposed to indicate the paint is too thick to measure. It can also indicate an issue with the probe itself or a calibration error.
So what does it mean if the paint is too thick to measure? Here are a few possible interpretations of the reading.
- Red Flag for Thickness Consistency: Getting an “overflow” reading after accurately measuring other panels can be a red flag as it indicates a significant difference in thickness across the vehicle body, which in turn indicates an inconsistent painting process. That means you may need to be more thorough in measuring the paint.
- Body Filler: An “overflow” reading can also be an indicator of body filler work, which typically uses thicker paint layers to cover damage.
- Overspray: In some cases, an “overflow” reading can be the result of overspray from hard-to-reach areas during the painting process, contributing to thicker paint layers in certain areas.
Flipping Display Reading
Sometimes when you’re reading a vertical panel it can be hard to read the display as the reading is upside down from your perspective. Here’s how you can flip the display for easier reading:
1. Power the gauge on. You will see a green light indicator blinking in the upper right hand corner. Continue holding the power button down.
2. The gauge will cycle through the operation menu.
3. After the display reads “SWITCH FONT”, the measurement will appear on the screen and you will hear a beep.
4. Let go of the power button. The display will now be flipped 180 degrees.
Case Study: 1969 Dodge Charger SRT w/ Custom Basecoat/Clearcoat Paintwork
To best show how a paint thickness gauge works in practice, I am including a “case study” to show my process for inspecting the custom basecoat/clearcoat paint on a 1969 Dodge Charger SRT prior to paint correction.
This first section of paint measures 1553 microns, or 61 mils for comparison. That is on the thicker side, which makes sense because this is a custom paint job and custom jobs will always tend to give thicker readings because more paint is sprayed on the car.
Factory paint, on the other hand, is on the thinner side because mass production requires efficiency, meaning less paint is applied to save money and speed up the manufacturing process.
As you can see, moving the gauge over just an inch returns a reading of 1776 microns or 70 mils.
Moving the gauge a few inches forward, on the other hand, gives me a reading of 964 microns, or 38 mils.
This goes to show custom paint jobs can range wildly in coating thickness over the various body panels. This is normal.
Go or No Go?
While I did see a wide range of measurements while gauging the various major body panels, they were all on the thick side. That gives me the confidence to use a compound if needed to remove the deepest defects.
That being said, just because I can doesn’t mean I should. Even if you are a “go” to use an aggressive compound, you should still do a test spot to see if that would even be necessary to remove the defects.
The Importance of Doing a Test Spot After Measuring
Measuring the thickness of a car’s paint tells you whether aggressive correction is possible.
Testing your paint correction process on a car’s paint tells you whether aggressive correction is necessary.
Always follow your measurement with a test spot before rushing to use heavy compounds. The big picture idea here is that you are dialing in a process that removes the defects to your satisfaction using the least aggressive means to leave as much paint on the car as possible.
Remember, factory clearcoats are thin to start with, so you always want to err on the side of caution. The more paint you leave on the car, the better the chance the entire paintjob will last for the mechanical lifespan of the car.
Explaining Paint Thickness with the Post-It Note Analogy
I grew up always using mils as my unit of measurement when measuring paint coating thickness. Years ago, in an effort to teach people just how thin FACTORY paint is on modern cars, I did some measuring and wrote an article called Clearcoat Paints Are Thin by Mike Phillips.
The article explained that on average, the factory layer of clearcoat paint sprayed over the basecoat or colored layer of paint is approximately 2 mils thin. Knowing a common Post-it Note is approximately 3 mils thin and knowing the average person has access to a Post-it Note, I found it easy to open people’s eyes as to how thin the factory paint is on their modern vehicle.
I can either hand someone a Post-it Note or tell someone to find a Post-it Note and then hold it between their thumb and index finger and it’s pretty easy for the average person to wrap their brain around just how thin the Post-it Note is.
Then after telling a person a Post-it Note is 3 mills (and they are feeling 3 mils between their fingers), I then tell them the factory clearcoat layer of paint on their car is THINNER than the Post-it Note, this drives the point home in a very easy to understand manner.
And then our conversation tends to go to the topic of always using the least aggressive product and process to get the job done, i.e., do the paint correction step, BECAUSE the paint on their car is so THIN.
Now fast-forward about 15 years and like most seasoned professionals, I have switched over to measuring paint using microns instead of mils because as stated at the beginning of this article, the micron unit of measurement is dramatically more accurate and thus a safer unit of measurement to use before doing any paint correction steps.
Final Thoughts…
Whether you’re an enthusiast do-it-yourselfer primarily detailing your own cars, or if you’re a professional detailer working on customer cars, it’s a good idea to own a quality PTG or Paint Thickness Gauge to measure the paint thickness before doing any paint correction to the car.
Because most paint gauges measure TOTAL paint thickness, that is the gauge reads all layers of coatings over the top of the substrate, this means you can never know the actual thickness of the top coat, the layer of paint you’re going to be working on.
Thus a PTG is what I call the Go or No Go Decision Maker. It’s a BIG PICTURE INDICATOR. It gives you an idea of the total thickness of the paint on the car in front of you. I know we all want a PTG that measure the individual layers of paint, including the,
- Ecoat
- Primer
- Basecoat
- Clearcoat
And while there are coating thickness gauges that can read and measure the different layers, these types of gauges tend to be very expensive and out of the budget of most people. Even if you had one, the TIME it would take to measure and document the paint coating readings over an entire car would be too time-consuming.
A Better Plan of Attack
I’ve been running a polisher over paint since the 1980s. That’s like four decades as I type today. The majority of the time I never had a PTG and in my early years, before the Internet, I didn’t know they existed. Yet I have been able to buff out THOUSANDS with no problems without using a PTG before doing any sanding or paint correction.
How Did I Do It?
I used quality products. I matched the pad to the process. I used the right tool for the job and used good technique and a little common sense. Some might say I got lucky.
Should You Buy a Paint Thickness Gauge?
If you’re really into taking care of your own cars, then yes, I would recommend purchasing a quality, and trustworthiness paint thickness gauge. When it comes to working on modern factory paint, any help you can get to understand how thick or how thin the paint is before you start any paint correction steps is a benefit and if nothing else, will give you some level of peace-of-mind.
If you’re a professional detailer, the answer is a hard yes. Not only will using a paint thickness gauge let you know if you’re working on super thin paint and thus prevent you from getting too aggressive and possibly buffing through the clearcoat, but it also shows you are a true professional that knows their craft.
My Personal Offer to Help
I’ve been answering car detailing questions professionally since 2002 and moving forward, if after reading this article you have any questions about polishing and waxing, or how to use an AIO, my contact information is below, reach out to me, I’m always happy to help.
Would You Like to Learn How to Machine Polish Like a Pro?
- Would you like to take your detailing skills to a higher level?
- Do you find you learn better by doing versus sitting in a chair looking at a PowerPoint presentation about car detailing?
Here at Dr. Beasley’s O.R., (Operating Room), in sunny Stuart, Florida, we teach the most hands-on car and boat detailing classes on planet Earth. And we prove it too. You can find write-ups on the Dr. Beasley’s Blog that photo-documents just how hands-on our classes are plus you’ll see the kinds of cars and boats you’ll be training on.
Click the link below to find the dates, locations, topics, and prices for all upcoming detailing classes.
Upcoming Detailing Classes – Prices – Info – Topics – Locations
And if you ever have any questions, always feel free to reach out to me personally via text, phone call or email.
I hope to see you in a future detailing class!
Sincerely,
Mike Phillips
Chief Education Officer for Dr. Beasley’s
Mike@drbeasleys.com
760-515-0444
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