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
Accuracy
+ / - 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.
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.
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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