Clear Coat
A clear coat system consists
of one or more primer layers, a flat colour layer and a glossy, clear top
layer. The primer is a corrosion
inhibitor and a bonding agent for the bare metal and the colour layer. It prevents corrosion and provides a stable
substrate for the colour and clear coats.
The colour layer is applied to the primer and is typically very
thin. Its only purpose is to provide
colour. The clear coat is two to three
times the thickness of the colour layer, adding to the appearance of paint
depth and offering additional protection.
Manufacturers also use ultraviolet-light-blocking
technology in their
clear coat systems for protection against photo degeneration (sun fading).
Modern
Isocyanate resins (clear coat) 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, the clear coat that has a thickness of ~25.4 µ (micron). As a point of reference a
sheet of copy paper is 89 µ.
A
micron (µ) is a metric unit that equals one millionth of a meter, or 1/1000 of
a millimetre. A micron is much smaller than a Mil. A human hair is about 2 Mil
(50 µ) thick and individual bacteria are 0.1 mil (2.5µ) in size. There are 25.4
millimeters in an inch and a micron is 1/1000 of a millimetre.
Using
the micron (metric) measurement system gives you a much better idea of paint thickness
as the numbers used are so much smaller. Most detailers are or should be aware
of how thin clear coat paint is and it puzzles me why people over-polish paint
causing it to fail prematurely
Abrasive Polishing
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.
There are two considerations;
how much clear coat and how much ultra violet protection can be removed, they
are not interchangeable. The following are the maximum allowable clear coat
reductions the major USA car manufacturers will allow before the paint warranty
becomes void; 0.3 Mil (7.5 µ)
Order of Magnitude (~ Thickness)
·
Standard printer copy paper 7.5 µ (0.
3 Mil)
·
A standard sandwich bag 28µ(1.1 Mil)
·
Dollar ($1) bill
73.5µ(2.9 Mil)
The 0.2 Mil (0.5 µ) is 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.
Wet-sanding, compounding and
polishing the amount of paint removed with a mild abrasive was ~ 0.1 mil (0.25
µ) this does not mean that the clear coat will fail if you remove 0.5 Mil (12µ)
There are many factors
involved and each car plant may have a unique paint system that is designed for
their specific needs and the thickness varies from one plant to another so
there is no way to make such generalizations. One thing is for sure that if you
keep removing clear coat, at some point you will experience strikethrough
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 manufacturer) you would only be able to
remove <12µ without voiding the paint
warranty and possibly having a re-paint.
Most light surface marring
is ~1.27 µ (0 .05 Mils) a surface scratch that can catch your fingernail
is ~1.01 µ (~ 0.04 Mils) Using a medium abrasive polish and a rotary
polisher will remove approximately ~ 2.5 - 3µ (~
0. 98 – 0.12 Mil) from the paint surface. To remove a scratch you need to level
the paint to its lowest part, so if a scratch is 1 µ
that’s the amount of paint (and UV protection) you need to remove to eliminate
it. Note: 25.4 µ
(micron) = 1 Mil
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)
PPG’s CeramiClear clear coat is the first
automotive clear coat to use nano particle technology in the final coating
applied to car bodies, protecting the colour coat while providing a durable,
glossy appearance. With the help of the nano-technology developed at the
beginning of the 1980s, scientists have been able to alter the molecular
structure of the binding agent and integrate tiny, microscopic ceramic
particles. These each have a diameter of less than 20 nanometers, which makes them
tens of thousands times thinner than a human hair.
During
the electrostatic paint application process, it is sprayed just like a Melamine
or Silane 2K clear; the key is what happens during the cross linking or curing
of the clear. The hard "ceramic" particles rise to the top, just as
the ultra violet (UV) inhibitors do, and concentrate there and the binding
agent particles float around freely at first in the liquid paint.
The
clear coat is the coating layer that forms the last interface to the environment. It carries the biggest part of the
technological performance and must be able to resist ultra violet radiation,
environmental etch, bird droppings, car wash machines and other outside
influences.
Be cognizant that clear coat
thickness and composition will vary in accordance with the OEM assembly plant,
paint specification, paint type (solvent or waterborne) and paint supplier and
any additives used i.e. ultra violet stabilizers, Isocyanate hardener and
catalysts
Base Coat Clear Coat – two
stage paint; base (colour) coat and clear coat were adopted as an automotive
industry standard in 19982, clear coat paint was originally used to
protect metallic paints that are subject to oxidation and provide depth of
colour. BC_CC paint systems do not oxidize in the same way as single stage
paint does, but they are subject to clear coat failure. They are applied over
the primer surfacer and covered by the clear coat layer to protect it from the
environment.
There exist three main base
coat systems in the paint shops of the automotive industry worldwide: medium
solids (MS) high
Solids (HS) Water-based
(waterborne) (WB) paints are essentially low solids
paints (up to 60% waterborne solvent), but they are legal because deionized water is used as the solvent, as opposed to volatile organic compounds (VOC).
The main purposes of the
solvent are to adjust the curing properties and viscosity of the paint. It is
volatile and does not become part of the paint film. It also controls flow and
application properties, and affects the stability of the paint while in liquid
state. Its main function is as the carrier for the non volatile components
Density (or specific
weight); different materials usually have different densities, so density is an
important concept as less dense fluids float on more dense fluids if they do
not mix, causing it to rise to the top (we have Archimedes to thank for this
discovery)
If the average density of an object is less
than that of water, which is 1.0 g/ml, it will float in and if its density is higher
than water's it will sink. Most organic solvents have a lower density ~0. 8
g/ml than water, which means they are lighter and will form a separate layer on
top of water.
Benzotriazole is fairly
water-soluble, not readily degradable and has limited sorption tendencies,
which is used in some paint formulations to provide ultra violet protection. It
has a specific gravity of 1.17 g/ml, which is heavier than water (0.98 g/ml)
and much heavier than solvent (0.80 g/ml)
There is ultra violet (UV)
protection all the way through the paint, vehicle manufacturers also add ultra violet
protection to the base coat to minimize the transfer due to density, the majority of it migrates to the top as the
paint cross-links, along with the
thinner solvents and particulates, the paint is also somewhat softer below this
level. The amount of migration will vary
with the formulation of the paint, and which ultra violet protection chemical
is used.
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 ultra violet inhibitors).
Be cognizant that ultra
violet protection 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 ultra violet protection, its long-term durability and / or the
paint warranty
Photo degradation
[:
decomposition of a compound by radiant energy] a common reaction is oxidation.
Today’s water-based products
have a number of obvious environmental benefits, but some are more susceptible
to photo degradation (fading) over time, a significant drawback. In addition,
because of ozone depletion, higher levels of solar ultra violet (UV) radiation
now reach the surface of the earth. This further contributes to the rate of
fading.
Polymers use in auto paint,
plastics and vinyl, where they are routinely exposed to sunlight; the UV
radiation adversely the mechanical properties of these materials, often causing
structural failure, which limits their useful life.
Paint Thickness
A
paint thickness reading of > 100 µ
(Microns) is reasonably safe for polishing.
80-90 µ, I wouldn't use anything
stronger than >2000 grit polish, 70-80 µ >2500, 70
µ use a glaze. The readings tend to vary from panel to panel and are
thinner towards the panel edges and any seams.
·
200µ + can be expected on older cars that have been hand painted or a
re-painted vehicle
·
100 – 200µ - average paint thickness
·
80 – 100 µ - thin paint
· > 80 µ - very thin paint
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 and etc that may
affect the amount of paint and ultra violet protection removed
If
you have reservations about the amount of paint surface removed or the amount
of paint 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 / or UV protection, 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
Order of Magnitude (Thickness)
·
Standard printer copy paper 76µ (3 Mil) thick.
·
A standard sandwich bag 28µ(1.1 Mil)
·
Dollar ($1) bill
73.5µ(2.9 Mil)
These numbers are offered as a guide only, as there are too many
variables to provide any more than an approximation.
Notes:
1. The elongation
(elasticity) of paint enables it to move in tandem with the metal as it expands
and contracts due to environmental temperature fluctuations; for this reason
note the paint temperature when taking readings as they can vary in accordance
to the surface temperature.
2.
Measure
your paint thickness in a very cold environment, then measure it when the paint
surface is hot to the touch, you may find it varies by as much as a 2µ(microns)
Paint Insurance
The newer coatings available
like synthetic polymers are a
cross-linking thermoplastic, its cross-linking
process attaches the polymer with covalent bond that becomes part of the
surface of the material it is attached to, which in effect becomes a secondary
protection for the clear coat, in fact a relatively inexpensive (when compared
to repainting) renewable sacrificial
coating.
Silica
(AQuartz) or reactive resin hydrophobic coatings (OPT
Opti-Coat™); think paint sealant that has greater durability and scratch
resistance (9H) something that also provides a self-cleaning protection, with a
durability on a timescale measured in years rather than months, these coatings
add a measurable protection of 2-3 µ microns to the clear coat.
Opti-Coat™ is not a nano
particle; it is rather a pre-polymer that cross links and forms a continuous
film on the surfaces it is applied to, similar to a single component isocyanate
that forms a clear coat finish. It
is very resistant to alkaline car wash concentrates as used by car wash spas
and tunnel wash companies
The coating is very low
maintenance and requires cleaning less often than conventional paint protection
products. Provided the surface is kept free of abrasive grime, bird excrement
or other acids, the coating should last for around two years. Coating longevity
will be improved if the paint surface is cleaned on a regular basis, and only
with either normal pH car care concentrate shampoo or a citrus degreaser.
After Care
To enable a vehicle to
maintain its value original paint that is in good condition is an asset. People
are keeping their vehicles an average of nearly nine years. Making you client
aware of how to wash and dry a paint surface while inflicting the least amount
of damage will help to avoid the need to overly polish the clear coat to remove
scratches and the subsequent loss of both clear coat and ultra violet paint
protection.
The main point of this
article is to inform detailer that there is a very limited amount of clear coat
and to keep them from over polishing and unnecessarily removing their clear coat
and ultra violet (UV) protection.
Providing this kind of value
added-value services will enhance your reputation and enable you to become the
source for both ethical service and high quality work. As with the successful sale of any product, educating the
customer is the key.
Bibliography
1.
UV Protection and Coatings for Plastics in the
Automobile Industry - Paint &
Coatings Industry (PCI) - http://www.pcimag.com/articles/uv-protection-and-coatings-for-plastics-in-the-automobile-industry
2.
Protecting UV-absorbing Clear Coats from Sunburn," Polymers
Paint Colour Journal, February 2000
References
1, Distribution of Stabilizers in Multi-Layer-Coatings and Plastic
Coatings - 3rd International Coatings for Plastic Symposium, Troy, MI, June
2000.
2, Potential Reasons for Yellowing of Coatings over Plastic Substrates - 4th International Coatings for Plastic Symposium, Troy, MI, June 2000
3, Protecting UV-absorbing Clear Coats from Sunburn - Polymers Paint Colour Journal, February 2000
2, Potential Reasons for Yellowing of Coatings over Plastic Substrates - 4th International Coatings for Plastic Symposium, Troy, MI, June 2000
3, Protecting UV-absorbing Clear Coats from Sunburn - Polymers Paint Colour Journal, February 2000
Bibliography
1.
The
Biology Project, Department of Biochemistry and Covalent (molecular) Biophysics
2.
The
University of Arizona, Revised: January 28, 2003
3.
The
Vinyl Institute Information
4.
The
Royal Society of Chemistry; Cambridge, 1995 Silicon-Containing Polymers
Richard, G. Jones
5.
Royal Society of Chemistry (RSC) Library &
Information Centre
6.
The
Chemistry of Polyurethane Coatings - Bayer
publication
7.
Glossary
of Chemical Terms - Faculty of Chemical Technology
8.
The
Basics of Silicone Chemistry - Dow Corning
9.
Basic
Concepts of Nanotechnology, History of Nano-Technology, News, Materials and Potential Risks
10.
SpecialChem4
Polymers
11.
Macromolecular
Chemistry and Physics
12.
National
Polymer Laboratories Newsletter /
Bulletins
13.
Sol-Gel
Science - The Physics and Chemistry of Sol-Gel Processing by C. J. Brinker and G. W. Scherer,
14.
Self-Healing
Polymer Coatings, Cho, S.H.Cho, S.R. White, and P.V. Braun
Information resource
1.
The
Center for the Polyurethanes Industry (CPI) of the American Chemistry Council
2.
Polymer Basics DoITPoMS - University of Cambridge
3.
Wikipedia
Dictionary - http://en.wikipedia.org/
4.
Encyclopaedia
Britannica Eleventh Edition - http://info.britannica.co.uk
I
would like to think that these articles become an asset to anyone who is new to
detailing and to professionals alike, as well as industry experts who seek to
advance their knowledge.
Hopefully 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.
I would appreciate it if you would share
this article as it helps other detailers further their knowledge.
Questions and/ or constructive comments
are always appreciated.
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