Film thickness can be measured using two units of measurement. The first and more common and standard measurement are called mil. A mil equals 1/1000 of an inch. A typical factory paint job will measure between 4.0 and 6.0 mils. The clear coat will generally measure between 1.5 - 2.5 mils



The alternative measurement is called a (µ) micron, which is a metric unit that equals one millionth of a meter, or 1/1000 of a millimetre. A micron (µ) is much smaller than a mil; there are 25.4 millimetres in an inch and a micron is 1/1000 of a millimetre.



I use the micron measurement because it`s so much smaller. One micron is roughly 1/80th of the thickness of a human hair (hair thickness depends on hair colour/ethnic origin etc) I always want to know where I am at all times in terms of paint removal. If you are going to get into heavy correction such as wet sanding, you will be quickly removing clear coat, and will need to keep track of how much is being removed, and will need to be accurate. Micron (µ) readings will constantly change as I sand; keeping me constantly informed of how much is being removed, and will need to be accurate. A mil reading will not change that rapidly during the process.



Before using a paint thickness gauge (PTG) you must understand the units of measurement used the relevance of the numbers shown and what you are actually measuring. Most paint thickness gauges (PTG) work by sensing the distance between the sensor and the metal base material, which means it`s measuring the (primer, base, colour, and clear coat) the total paint thickness. Paint polishing entails the removal of scratches from the clear coat, but the PTG shows the total paint thickness, so we need to know the average clear coat thickness.



As you can see, a PTG will only give you an approximation of the clear coat thickness; you need to take an educated guess as to how much each layer consists of. Using this as a guide to how much clear coat you can remove without compromising the paint system is much better than polishing ‘blind ‘its value is in diagnosis as opposed to guesswork



Accuracy and Types Available



+ / - 2µ (micron) based on measuring the magnetic force and determining how thick of a non-conductive coating is covering the substrate, or sonic resonance (ultrasound) instead of magnetic forces, thus its ability to measure on non-conductive substrates.



The PTG unit’s calibration must be checked before use if you’re going to rely on the information. They usually come with testing ‘metal shims’ that have a thickness marked on them so you can check the readings obtained and recalibrate if necessary



There are models that measure only ferrite containing alloys and others that measures alloys of all metals (aluminium panels for example), and the top model that measures any type of surface



Clear Coat Thickness



Modern clear coat paint finishes are so good today that they lull people into thinking that vehicle paint has protection and shine when in reality there is not really much there, other than the clear coat that has a thickness of ~25.4 µ. As a point of reference a sheet of copy paper is 89 µ.



The following are the maximum allowable clear coat reductions the major USA car manufacturers will allow before the paint warranty becomes void; Chrysler- 0.5 Mil (12µ) Ford – 0.3 Mil (7.5 µ) GM – 0.5 Mil (12µ) (Source - Automotive International)



The clear coat provides gloss plus physical protection from the elements, including ultraviolet (UV) radiation, which is in the upper level of a cured clear coat. Most car manufacturers will only allow ~ 25% of the clear-coat thickness to be removed without voiding the paint warranty and long-term durability problems becoming an issue. That means that if you started off with 50µ of clear coat (this will vary by vehicle mfg) you would only be able to remove 12µ without voiding the paint warranty and possibly having a re-paint (Note: this may vary by vehicle mfg).



There is ultra violet (UV) protection all the way through the paint, but the majority of it migrates to the top of cross-linked clear coat along with the thinner solvents and particles, the paint is also less dense (softer) below this level. Therefore removing clear coat ultra violet protection is not a linear process; by removing a small percentage of the clear coat paint tends to remove a larger percentage of the ultra violet (UV) inhibitors.



With a clear coat thickness of ~49µ and knowing that most of the ultra violet protection is in the top 50% (~24.5µ); therefore, limiting UV protection removal to ~25 % means that approximately 6.125µ< can e removed before the ultra violet protection is compromised. Once you remove too much clear coat you`ll have no paint UV protection other than what you apply with a LSP (providing it contains UV protection.



Be cognizant that clear coat removal is not a liner process; and the first paint renovation will remove the most UV protection, therefore the above are probably conservative estimates. Two variables need to be established; how much clear coat is available and how much clear coat can be removed without compromising the paint systems long-term durability / paint warranty



Paint Removed by Polish or Compound



Using a medium abrasive polish and a rotary polisher will remove approximately 2.5 - 3µ (0. 1 Mil) from the paint surface, which is typically four passes at 1500-1800 RPM; however many variables such as polish/compound and speed / pressure used that may affect the paint removed)



If you have reservations about the amount of paint surface removed or the amount of paint coating remaining the use of a paint thickness gauge (PTG) is arbitrary. There comes a point when you must judge wither removing a scratch will compromise the clear coat and if so you’ll have to ‘live’ with the imperfection

.

Paint thickness will often depend upon the OEM paint specification, which can vary by vehicle assembly plant. It’s interesting to note that painters must now demonstrate proficiency with an electronic paint thickness gauge in order to become certified to perform paint refinish warranty work for General Motors Corp. (GM) vehicles



Mil - Micron

0.1 Mil = 2.5 µ (micron)

0.2 Mil = 5.0 µ

0.3 Mil = 7.5 µ

0.4 Mil = 10.0 µ

0.5 Mil = 12.75 µ

0.6 Mil = 15.0 µ



Using a Paint Thickness Gauge (PTG)



Using a paint thickness gauge gives a professional detailer credibility and also acts as an insurance against ‘break-though’ of a clear coat, which would entail an expensive re-paint and I suspect a very irate customer.



Original paint has a better resale value than a re-painted vehicle, plus they will lose the use of their vehicle for some days. The ideal setup would be a DFT Combo for the metals and a PosiTector 200 B/Adv for the composites



Using a medium abrasive polish and a rotary polisher will remove approximately 0. 1 Mil (3µ ) Range 0.8 – 1.1 Mil (20 – 28 µ) from the paint surface (typically 4 passes at 1500-1800 RPM) but there are many variables such as the abrasive grade of the polish or compound and speed and pressure used that may affect the paint removed) These numbers should be checked with a paint thickness gauge (PTG) There comes a point when you must judge wither removing a scratch will compromise the clear coat and if so you’ll have to ‘live’ with the imperfection.



A paint thickness reading of 4 Mil < ( 100 µ (Microns) is reasonably safe for polishing. 3 – 3.5 Mil ( 80-90 µ) I wouldn`t use anything stronger than > 2000 grit polish, 2.75 – 3.0 Mil (70-80 µ) > 2500 grit polish and under 2.75 Mil (70 µ) use a glaze. The readings tend to vary from panel to panel and are thinner towards the panel edges.



If you have reservations about the amount of paint surface removed or the amount of paint coating remaining the use of a paint thickness gauge (PTG) is arbitrary



These numbers are offered as a guide only, as there are too many variables to provide any more than an approximation.



Note: 1 µ (micron) = 0.001 mm, 1 Microns = 0.00004 Inches



• 200µ + can be expected on older cars that have been hand painted or a re-painted vehicle

• 100 – 200µ 4 – 8 mil - normal paint thickness

• 80 – 100 µ - 3 – 4 mils, thin paint

• 80 µ < - less than 3 mil, very thin paint



Ultra violet protection (UVR) a clear coat paint (isocyanate polyurethane) provides the colour and base coats with protection, and usually has UVR protection added in the final coating; this tends to migrate towards the upper level of the clear coat (this is why it’s important to check how much of the paint surface you are removing with an abrasive polish)



Removing more that 0.5 mil (12µ) of clear coat will cause premature paint film failure as ultra violet (UV) radiation protection percolates to the top of the clear coat. UV protection is a sacrificial / renewable component; this is due to the UV protection layer being degraded by exposure to the elements (sun, sand, road or sea salt, and etc) it is also water miscible, so it is imperative that you renew it and needs to be re-applied on a regular basis (dependent upon location climatic condition)



Usage



A paint thickness gauge for use in the detailing industry requires fine measuring resolution in increments of 0.1 mil (0.0001= inch) is typical. Therefore you should choose a paint gauge with a minimum 3% accuracy and a 0.1 mil (2 micron) measuring resolution. An electronic paint thickness gauge can be an invaluable tool, before buffing it’s always a good idea to check the paint film thickness to see how much of the paint film thickness available, check is periodically during the process and on completion to see how much was removed.



Simple yet effective - a laser pen taped to a camera tripod. This enables you to accurately pin-point an area of paint you are polishing and providing you don’t move the tripod you will be able to get a very good indication of paint removal after polishing



Take measurements with a thickness gauge and mark them on a template so that you have a reading of before / after polishing.



Vehicles left outside absorb infrared (UV-A) radiation and can experience temperature swings of + / - 50o; solvents, resins, polymer and other ingredients in paint expand when they are heated, so dependent upon paint surface temperature, you will obtain differing paint thickness readings.



Take readings on each panel left – right every 2- 4 inches, then drop down 2-inches and repeat. –



Paint Thickness Template - Detailing (PTG) Template picture by TOGWT - Photobucket



If you use a car outline template and mark the paint thickness, you can advise the customer of any potential problems and you’ll also have a permanent record of the customers paint surface, which can be used as a reference each time the car is polished.



Select substrate:



F - for ferrous metals (steel and cast iron)

N - for non-ferrous metals (aluminium, copper, brass, etc.)

FN - for all metal substrates, gage automatically recognizes the substrate and takes measurement 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 aluminium, bumpers and mirrors made of plastic.



Infrared (UV-A) affect on Paint Thickness



With an ambient temperature of 80.OF a dark colour horizontal surface subjected to infrared radiation (UV-A) a paint surface can be subjected to temperature swings of 60.OF + / - and can reach temperatures in excess of 150.OF.



The urethane used for automobile paint is classified as a semi-solid, micro structured permeable membrane, due to its micro-fissures, or pores’, being a thermosetting polymer it remains flexible, whilst retaining its tensile strength, to enable it to expand and contract to follow temperature fluctuations (elongation and contraction). So be cognizant that paint thickness readings will vary in accordance with ambient temperature.



It should also be noted that solvents, resins, polymer and other ingredients in paint expand when they are heated, so dependent upon paint surface temperature, you will obtain differing paint thickness readings

Measure paint thickness on a horizontal surface at temperatures below 50 degrees, and then measure the same spot at mid-day at temperatures above 80.OF.



After an hour measure the same spot, you will then obtain a thicker paint surface reading. Conversely with colder ambient temperatures you will then obtain a thinner paint surface reading. There are variables that will affect the paint readings, i.e. instrument accuracy, humidity, paint surface colour, reflectivity, etc.



PTG Technologies



Magnetic Induction

Using this technology the thickness gauge utilizes non-destructive principles when measuring coatings on Ferro-magnetic substrates such as Steel and Iron. The coatings must be non-magnetic such as; paint, enamel, aluminium, chrome, copper and brass.



Eddy-Current

This technology utilize a non-destructive principle when measuring coatings on non-magnetic substrates such as; aluminium, aluminium alloys, bronze, copper, brass tin and rustproof weak magnetic steels. The coatings must be electrically insulating such as; paint, enamel, plastics, anodized coatings and ceramics.



Ultrasonic

Used on fibreglass surfaces an ultrasonic transducer emits a high frequency sound pulse that travels into the coating via a coupling gel and reflects from ANY surface that is different in density. Coating thickness readings are obtained by measuring the time taken for the ultrasonic signal to propagate from the probe to the coating/substrate interface and back.



The travel time is divided by two and multiplied by the velocity of sound in the coating to obtain the thickness of the coating. In some circumstances, individual layers in a multi-layer system can be measured. An ultrasonic gage is able to differentiate between layers by the quality of the echo that is returned from the material boundary and the sensitivity of the detection method.



The quality of the echo is dependent on the how closely the materials are matched in “characteristic impedance (a function of a material’s density and the speed of wave travel through it) this is also known as acoustic impedance.



The closer two materials’ impedances match, the fewer echoes, as the sound propagates through them in the same way, conversely the greater the difference between their impedances, the larger the echo, there’s no reason an ultrasonic won’t work on metal substrates. They have their own characteristic impedance like any other material. In fact, ultrasonic measurement is often used for checking pipe wall thickness.



Note: The metal flake in automotive paint is aluminium and is therefore a non-ferrous metal (iron).



Accuracy



Some units provide test plates with the Mil or Micron thickness of the plates marked on them. If, for some reason, when you test your unit on these plates, which you should do before each use, if the reading is inaccurate they provide detailed instructions on how to re-calibrate the unit



General Information



Most OEM paint finishes range from 4 to 8 mils , 1 Mil=1000th of an inch (100 - 200µ microns) this will vary between manufacturers, for this reason, it’s also recommended that paint be measured when buffing since you can’t see how much paint is being removed during the buffing process. If an insufficient thickness of clear remains after polishing premature failures such as fading or delaminating may occur as the colour coat won`t be protected from ultra violet (UV) radiation and a warranty issue may develop in the future. Such risks can be alleviated by monitoring and minimizing the amount of top coat removed, which is best done through quick and easy measurements with an electronic paint gauge, and maintaining a record on a paint thickness template.



Water based paints Water-based automotive paint is classified as a semi-permeable membrane that has tensile strength and elongation (elasticity) to enable it to move along with the metal as it expands /contracts due to environmental temperature fluctuations



Paint is a simple mixture of ingredients including pigments, a binder and diluents, or thinner (in a water-based paint it is water; in a solvent-based paint, it is petroleum solvent).



Automotive paints utilizing modern paint technology using water-based high solid/low solvent urethane that for environmental reasons produce a much softer and more porous finish than acrylic or oil based high solvent content paint systems.



Using permanent pigmentations and colorants specifically designed for automotive applications (containing automotive-grade binders). Albeit a non-toxic (less than 0.01-percent VOC) water-based paint, there are no compatibility issues with major-brand urethane and polyurethane clear coats.



Paint Thickness by Vehicle Mfg



• German and Japanese cars: 4-5 Mils

• With the exception of: Lexus-White and Tan: 5-7 Mils,

• Lexus- Black and Silver: 4.5-6 Mils

• BMW: Silver: 6-7 Mils

• FORD: Approx. 5.5-7 Mils

• GM Cars and Trucks: Approx. 4.5-6.0 Mils, Jimmy’s and Blazers: approx 7 Mils

• Chrysler: Approx. 5.5-7 Mils (Note Sebring hoods are fibreglass)

• Dodge Cars: Similar to Chrysler Dodge Trucks and newer Ram Pick-ups: Cab approx. Mils and the boxes approx 9 Mils

• Jaguar: approx. 6-7.5 Mils (but they tend to vary)

• Jeep Grand Cherokee: These read approx. 6-8 Mils, Wranglers: approx. 4.5-5.5 Mils



Note- the above are provided for information only, you should always check the paint thickness before using an abrasive polish



Paint thickness will often depend upon the OEM paint specification, which can vary by vehicle assembly plant. It’s interesting to note that painters must now demonstrate proficiency with an electronic paint thickness gauge in order to become certified to perform paint refinish warranty work for General Motors Corp. (GM) vehicles.



How-To



Most paint thickness gauges work by sensing the distance between the sensor and the metal base material. That’s total distance, which would be the thickness of all layers (primer, base, colour, and clear coats) and any repainted material, etc. You have to take an educated guess as to how much each layer consists of.



A paint thickness gauge for use in the detailing industry requires fine measuring resolution in increments 0.1 mil (0.0001- inch) is typical. Therefore you should choose a paint gauge with a minimum 3% accuracy and a 0.1 mil (2 micron) measuring resolution.



An electronic paint thickness gauge can be an invaluable tool, before buffing it’s always a good idea to check the paint film thickness to see how much of the paint film thickness available, check is periodically during the process and on completion to see how much was removed.



Take measurements with a paint and coating thickness gauge and mark them on a template so that you have a reading of before / after polishing. Take readings on each panel left – right every 2- 4 inches, then drop down 2-inches and repeat.



Template - Detailing (PTG) Template picture by TOGWT - Photobucket



Using a paint thickness gauge gives a professional detailer credibility and also acts as an insurance against strike through of a clear coat, which would entail a re-paint and I suspect a very irate customer. Original paint has a better resale value than a re-painted vehicle, plus they will lose the use of their vehicle for some days.



The ideal setup would be a DFT Combo for the metals and a PosiTector 200 B/Adv for the composites.



If you use a car outline template and mark the paint thickness, you can advise the customer of any potential problems and you’ll also have a permanent record of the customers paint surface, which can be used as a reference each time the car is polished.