Polishing alleviates a myriad of paint surface problems. But use abrasive polishes wisely to maintain paint condition and to resolve surface damage problems. But know when to seek alternative methods of damage control and avoid over polishing with harsh, abrasive polishes. Original paint has a better resale value than a re-painted vehicle, plus the client will lose the use of their vehicle for some days.
Film thickness can be measured in two ways. 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
Approximate order of magnitude: standard printer paper or a dollar bill 80 µ (micron) (3 Mil), a standard sandwich bag 25.5 µ (1 Mil)
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
+ / - 2µ, 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 re-calibrate if necessary
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.
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 100 µ (Microns) (4 Mil) is reasonably safe for polishing. 80-90 µ (3 – 3.5 Mil) I wouldn't use anything stronger than > 2000 grit polish, 70-80 µ (2.75 – 3.0 Mil) > 2500 grit polish and under 70 µ (2.75 Mil) 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
Note: 1 µ (micron) is 1/1000th of a millimetre or 0.0393700787 Mil or 0.001 of an inch
Total Paint Thickness
• 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
These numbers are offered as a guide only, as there are too many variables to provide any more than an approximation.
As a professional detailer you should be aware of the limitations / consequence’s that limited paint thickness reading entail; and as a responsible professional you should advice your client
Ultra violet protection (UV) a clear coat paint (Isocyanate polyurethane) provides the colour and base coats with protection, and this is why it’s important to check how much of the paint surface you are removing with an abrasive polish
Note: A 2014 original equipment manufacturers (OEM) technical service bulletin (TSB) states that 0.3 Mil (7.5 µ) maximum clear coat removal from any marque vehicle This the maximum paint that they are allowed to remove on the assembly line at the factory during their paint sanding and polishing process to remove dirt nibs. This number is based on testing carried out at both General Motors (GM) and Chrysler testing centres.
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.
I hope the above article was informative. By having some understanding of the ‘What’ and ‘Why’ as well as the ‘How’ along with a little science to help you understand how the chemicals we use react, you can achieve the results you desire.
Questions and/ or constructive comments are always appreciated
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