Paint is a liquid, but as it
dries it becomes a solid and a gas (the solvent) that evaporates, leaving
behind the solid. The three primary type of ingredients found in all paints are
(1) Binder (film
former) (2) Vehicle (solvent) (3) Solutes (pigment).
The binder is the film-forming component of paint, it imparts properties such
as gloss, durability, flexibility, and toughness.
Many binders are too thick
to be applied and must be thinned. The thinner is also called the carrier
system or vehicle, because it makes it possible to transfer the binder to the
surface. The pigment provides the paint with colour. Some paints form by
solvent evaporation only, but most rely on cross-linking processes
One of the most often asked questions; “What polish / pad combination should I use to remove scratches?”
There are far too many
variables to state this accurately.
Diagnosis is the key, not guess work; (see
Test Panel Area)
before deciding on what products to use ascertain the paint surface conditions
and your goals for the paint finish. And then proceed with the least abrasive
product / pad combination
Differing density (hardness)
of paints react differently to correction, so before beginning to polish
measure the paint thickness (PTG) panel by panel to ensure that you do not
select an aggressive polish that will remove too much clear coat
Compiling a chart that lists
the consistency of automotive paint can only at best be a rough guide, as this
would necessitate listing by; manufacturer, OEM assembly plant, model year,
colour, new or aged, paint specification, and etc. To ascertain paint hardness
requires extensive experience working on a wide range of vehicles to have a point
of reference.
Identical vehicles of the
same make and model/year can react very differently to compounds/polishes/etc.
There are too many variables (paint spec, curing method, paint age, etc.)
Hardness
[In materials science, hardness is the characteristic
of a solid material expressing its resistance to permanent deformation. There
are three principal operational definitions of hardness: Scratch hardness,
Indentation hardness, Rebound or dynamic hardness]
Modern
clear coat urethane even though they are harder than lacquer paints, they still
scratch more easily. And because they tend to be harder it's more difficult and
time consumer to remove swirls and scratches. This is the main reason to take
vendor claims of ‘provides 9H hardness ‘for what it is – totally irrelevant.
How can a dense (hard) clear coat be so easily
scratched? Force acts through a body that has a surface area; if the
surface area is really small while maintaining an equal force, the pressure
becomes astronomical and the object under pressure capable of penetrating the
surface of an otherwise tough material. (Newton's
third law of motion)
Hardness is dependent on ductility,
elasticity, plasticity, strain, strength, toughness and viscosity; suffice to say it
is extremely important that paint hardness is properly taken into account when
polishing, because if it is not then inappropriate polishing products and pads
may be selected that either remove too much paint or turn out to be completely
ineffective
Hard and soft are relative
terms; you can forces are the subject of Newton's third law
of motion; the law of reciprocal actions [: to every action there is an equal and opposite reaction]
If you press down on your
paint finish with your palm it feels really hard and tough, but that’s because
the surface area of your palm is relatively large and what you’re actually
feeling is the resistance of the steel underneath the paint. Try pressing your
thumb nail into the paint with the same amount of force you used with your
palm, if you dare.
Scratch resistance can be
related to higher cross-link density and elasticity of the polymer network The
theory is that a dense cross-linked (hard) paint provides better protection
from scratches and stone chips, whereas a less dense (soft) paint absorbs the
impact is not based upon scientific fact.
In solid mechanics, solids generally
have three responses to force, depending on the amount of force and the type of
material:
·
Elasticity—the ability to temporarily change shape, but return to the
original shape when the pressure is removed.
·
Plasticity—the ability to permanently change shape in response to the
force, but remain in one piece
·
Fracture—split into two or more pieces.
Surface friction
Surface friction
Automotive paint surfaces contain
microscopic peaks and valleys, much like the profile of a mountain range. These
irregularities are known as capillary structures; there may be millions of
these defects per square inch. Contaminants such as fine dirt, minerals, and
pollutants are drawn into the voids where they are extremely hard to remove.
The filling of the capillary structures
with micro particles produces a hydrophobic surface. (a hydrophobic surface is
one that repels moisture.) In contrast, a hydrophilic surface is one that absorbs
moisture.
The potential of a surface to absorb or repel moisture is based on
many factors, including: temperature, relative humidity, material homogeneity,
and static electricity. Surface roughness is also a major factor; the rougher
the surface, the higher the spreading rate or attraction for water. The
smoother the surface, the more repellent it is to moisture (Lotus
Effect) and it lessens the chance of surface
marring.
Paint density
Paint / clear coat hardness, or more
accurately, density, can be affected by a number of factors including the
composition of the clear coat used (i.e. single, dual component or powder
coating) and how it is applied. There is a trade-off at the manufacturing stage
between scratch resistance and gloss levels vs. oven drying time and temperature,
along with the quality of the isocyanates and the products designed to add
gloss and accelerate paint hardening times.
Low
cross-link densities raise the viscosities of the polymer (soft). Intermediate cross-link densities transform gummy polymers into
materials that have elastomeric properties and potentially high strengths. Very
high cross-link densities (hard) can cause materials to become very rigid with a high gloss
Paint density is determined
primarily by OEM paint specification requirements, the variables are; the type
of resin used, the type of hardeners and other additives, types of solvents
used and the paint drying process used.
Somewhat
surprisingly, paint hardness actually varies significantly between different
makes of car, and also in some cases between different ages of the same make
and model of car. The reasons why this is so being quite complicated, but
suffice to say it is extremely important that paint hardness is properly taken
into account when polishing, because if it is not then inappropriate polishing
products and pads may be selected that either remove too much paint or turn out
to be completely ineffective.
A
unique aspect of polyurethane chemistry is that the hydrogen bonding acts as an
additional crosslink, but also allows thermoplastic flow, which helps the paint
surface to retain its elasticity and its tensile strength to relieve mechanical
stress.
The
basic structure of a polyurethane clear coat features a soft segment (polyol or
tetra methylene ether) which gives it flexibility and elasticity. There is also
a hard segment (polymerization) that has high urethane density, which gives the
coating hardness and tensile strength
In polymer chemistry, when a
synthetic polymer is said to be "cross-linked", it usually means that
the entire bulk of the polymer has been exposed to the cross-linking method.
The resulting modification of mechanical properties depends strongly on the
cross-link density
•
Each vehicle assembly plant uses different clear coat paint from
one of three major paint suppliers; PPG, DuPont, and BASF products and each of
these companies have a range of several differing paints
•
Most vehicle manufacturers will issue a painting specification
that denotes amongst other things; a paint to hardener ratio, dependent upon
quality control this spec may or may not be strictly adhered to
•
Some vehicle manufacturers have more than one plant assembling
the same model of a vehicle; each plant will often use a different supplier for
the clear coat paints.
•
Each assembly plant may elect to use one of several OEM paints
from PPG, DuPont or BASF clear coat product lines.
This is also true of imports assembled in the US; usually one
paint supplier is approved for all plants; however, each plant may modify the
application/bake process in order to meet production demand. This may also
affect which has the harder or softer clear.
There are other factors that
will have an effect; the composition of the clear coat used (single, duel
component, or powder) this generally reflects the trade-off the OEM is prepared
to accept between scratch resistance and gloss level considered acceptable,
oven drying time and its temperature, quality of the quality of the isocyanates used, relevant age (i.e. how long ago was the paint
applied) spot panel repairs (refinish) that are carried out either at the
assembly plant or the rail head or port of entry. Paint density (soft paint)
could also be caused if it was polished before the paint had time to fully cure
To date only water-based
paints are used for the base coat, the clear coat is still a VOC compliant, low
ratio solvent- based paint, which will be denser, dependent upon the quality of
the isocyanates used. There are also a
couple of other factors; OEM use a different resin (Melamine) which must be
heat cured (300.0F) which produces a denser (hard) paint that tends
to be more difficult to polish, whereas body shops use an acrylic resin based
clear coat.
Paint can be very complex
and yet really interesting; in polymer chemistry, when a synthetic polymer is
said to be cross-linked, it usually means that the entire bulk of the polymer
has been exposed to the cross-linking method. The resulting modification of
mechanical properties depends strongly on the cross-link density.
This is how tight or dense
the paint matrix becomes after is it fully cured, think of a paint matrix like
that of chicken wire mesh, a small mesh (dense) larger mesh..., I think you get
the analogy. Hard paint has a dense cross-link density, whereas low density
(soft) paint has a less cross-link density.
Specific
gravity
[: the ratio of the density (mass of a
unit volume) of a substance to the density (mass of the same unit volume) of a
reference substance] using water as a reference
it’s SG is 1.00 at 20. OC, a substance with a relative density (or
specific gravity) less than 1.0 will float. Relative density (RD) or specific
gravity (SG) is a dimensionless quantity, as it is the ratio of either
densities or weights
Paint
density - fumed
silica is often used as a hardener; it has a low specific gravity and rises to
the upper layers of the paint. This is part of the reason for clear coats being
‘soft’ below 7.5 µ (0.3 Mil) fumed silica paint hardeners are evenly distributed
throughout the clear when applied, however, as part of the curing process; the
majority migrate to the top of the clear coat. This is why many paint
renovation detailers observe a less dense (softer) clear after the removal of
12.5 µ (micron) or 0.5 Mil.
The reasons
why this is so being quite complicated, but suffice to say it is extremely
important that paint hardness is properly taken into account when polishing,
because if it is not then inappropriate polishing products and pads may be
selected that either remove too much paint or turn out to be completely ineffective.
Scratch resistance - can be related to
higher cross-link density and elasticity of the polymer network. Low
cross-link densities raise the viscosities of the polymer (soft). Intermediate cross-link densities transform gummy polymers
into materials that have elastomeric properties and potentially high strengths.
Very high cross-link densities (Hard)
can cause materials to become very rigid with a high gloss
The scratch
hardness of organic paints / coatings is greatly influenced by factors such as
the nature of the metal substrate, the thickness of the coating and the
temperature and humidity of the surroundings. Little information is available
about the effect of these factors on the scratch hardness and how this property
is affected during the course of weathering. Tests were conducted using a
power-driven automatic Erichsen scratch hardness tester, on paint coatings
air-dried for 48 hours and then aged in the laboratory for various periods over
a span of 300 days.
It was found that some coatings which showed a fairly low
scratch hardness during initial testing attained a significantly greater
hardness during the course of ageing. The thickness of the coating had no
significant influence on the hardness during the early periods of air-drying.
However, with aged coatings, scratch hardness was observed to increase
significantly with the thickness of the coating.
Dense paint is very
resistant to correction; some paints will correct with a single pass and look
incredible, ready for wax even. Others will correct just as easily but leave a
hazy appearance that needs to be cleaned up with a less aggressive polish and a
soft pad.
The best way to detail
paint, wither it is hard or soft (dense or less dense); is by using the least
aggressive pad / polish combination followed, if necessary by an incrementally
more aggressive approach until you find what works.
The best way to detail
paint, wither it is hard or soft (dense or less dense); is by using the least
aggressive pad / polish combination followed, if necessary by an incrementally
more aggressive approach until you find what works.
•
Dense (Hard)
paint - paint requires a more aggressive abrasive polish and takes more time to
correct. But a dense (hard) paint
makes it easier to get that perfect gloss. PPG's Optech™ or CeramiClear™
respond better to polishes / compounds formulated for this type of paint
density.
•
Less dense (Soft)
paint- responds to abrasives more readily and makes polishing easier, but it’s
easier to induce surface marring.
•
Water-based paint is generally more porous and less dense
(softer)
•
Solvent-based clear coat paint is usually denser (harder) than a
water-based paint and not as porous
As always generalizing
carries a degree of risk as a panel(s) may have been re-painted. Before
commencing polishing do a test panel on the car, once you have achieved the
desired results with your selected polish / pad combination proceed
PPG's Optech™ or
CeramiClear™ respond better to polishes / compounds formulated for this type
of paint density.
Paint is graded by its density by a Durometer;
one of several measures of the hardness of a material. Like many other hardness
tests, it measures the depth of an indentation in the material created by a
given force. Clear coats can be made with harder or softer densities and that
will also determine its scratch resistance.
The enamel paint finishes on
vehicles from the 50’s and 60’s era were as tough as porcelain. But rightly due
to environmental concerns, that high percentage petroleum based paints have
been generally superseded, resulting in the softer water-based paint finishes
of today and the unavoidable orange-peel seen on many new and re-painted
vehicles.
Today’s paints,
unfortunately, rank somewhere near the bottom of the Mohr’s scale of hardness,
especially single coat black / red paint the exception being white single stage
and PPG's Optech™ or CeramiClear™,
when compared to all the materials your paint can possibly come in contact with
(always bear that in mind).
Mohr’s Comparative Hardness Scale
An
adaptation of that hardness scale (1 / 10)
1. Talc
= 1
2. Carbon
Black [black paint pigmentation] = 2
3. Glass
= 6
4. Titanium
dioxide [white paint pigmentation] = 7
5. Diamond
=10
It is extremely important
that paint hardness is properly taken into account when polishing, otherwise
inappropriate polishing products and pads may be selected that either remove
too much paint or turn out to be completely ineffective
Diagnosis is the key, not guess work; before deciding on what
products to use ascertain the paint surface conditions and your goals for the
paint finish. And then proceed with the least abrasive product / pad combination
Pencil Hardness
The Pencil Hardness Test
provides a simple method to test the scratch hardness of coatings. In this
test, pencils in a range of 6B to 8H hardness-grade are used. Pencils graded
using this system is used to measure the hardness and resistance of varnishes
and paints. The resistance of a coating (also known as its pencil hardness) is
determined as the grade of the hardest pencil that does not mark the coating
when pressed firmly against it at a 45-degree angle.
Select a pencil and make a
line about one-inch-long on the paint sample. If the pencil leaves a scratch,
then take the next softer pencil and do the same thing. The number of the first
pencil that you use after you have made a scratch in the coating that doesn't
leave a mark is considered the "pencil hardness" of the coating.
(There are some coatings that are so hard that even the 9H pencils will not
scratch them. All of those coatings get a 9H pencil hardness rating to
designate their hardness.)
Pencil Hardness for Common Coatings
·
Catalysed polyurethane: 7H
·
Catalysed modified acrylic polyurethane: 4H
·
Catalysed acrylic polyurethane: 2H
·
Water-based polyurethane: 3H
·
Water-based urethane/isocyanate catalyst: 2H
·
Low-VOC lacquer: 3H
·
Low-VOC catalysed lacquer: 2H (24 hours)
·
Urethane/nitrocellulose lacquer: F (24 hours)
·
Clear shellac aerosol: 3B
·
Polyurethane/nitrocellulose aerosol: HB
Hard / Soft Paint Correction
Experienced detailers can
evaluate paint condition quickly and accurately but they still perform a test
spot to make sure they choose the right methodology. Experience with different
OEM paint density and colour will teach you that no two vehicle paints, even
the same marque and paint colour are the same. Paint correction is a complex
process with many variables; polisher, type and abrasive level of pads and
polish or compound, speed and pressure used, and etc.
Dense
(Hard) paint - is very resistant to correction; if you see very little
change on your ‘test’ providing you are using an appropriate polish / pad
combination and proper technique and you’re seeing very little correction
you've got fairly hard paint.
When
dealing with less dense (soft) paint, your ‘test panel’ looks far from perfect
most neophyte’s are under the impression that they need to get more aggressive.
Nothing could be further from the truth, even though the finish may not look
all that great, it is an improvement from where they started. Usually a less
dense or intermediate density (soft or medium soft) paint will result in really
bad hazing.
Really
dense (hard) paint (DuPont's SupraShield™, PPG's Optech™ and
CeramiClear™) - will rarely haze; it’s usually very
unresponsive but will show signs of improvement after a few passes
Some paints will finish with
just a single pass; others will finish with a ‘haze’ that requires that you
'finish' with a less abrasive pad / polish combination. There are some paints
that have a very low
density (very soft) paint that allow swirls and micro marring, etc.
to be removed easily but it just appears dull and flat no matter what you do. A
very gentle approach is called for in those cases, requiring a uniformly
applied pressure across the pad / backing plate
Does paint density (hardness) affect its
scratch resistance?
Many coating products claim
an obtained surface hardness of 9H or 3 Gpa+, but this has little if anything
to do with scratch resistance. Scratch resistance can be related to higher
cross-link density and elasticity of the polymer network.
So how can a dense (hard) clear coat be
so easily scratched?
Force acts through a body
that has a surface area; if the surface area is really small while maintaining
an equal force, the pressure becomes astronomical and the object under pressure
capable of penetrating the surface of an otherwise tough material. (Newton's third law of motion)
That’s why a micro fine
thread that is twice as fine as silk and a 100 times finer than a human hair,
in an otherwise soft towel will scratch your paint. And the same reason a
mosquito can penetrate a rhino hide with its proboscis (stinger).
This can best be summed up
as: your car’s paint finish, though it may feel hard, is actually rather soft.
Nearly everything else that comes in contact with it, though it may feel soft,
is harder by comparison and will scratch it with enough pressure – sometimes
just the slightest of pressure.
Test Panel Area
Diagnosis is the key, not guess work; before
deciding on what products to use ascertain the paint surface conditions and
your goals for the paint finish. And then proceed with the least abrasive
product / pad combination
Modern automotive paint
systems of whatever density (hardness) pose unique challenges to the detailer,
to determine the correct polish/and pad combination, machine methodology. This
requires a ‘test panel’ to see how the selected polish set-up responses to the
paint, and wither it will remove the surface scratches
Select a ‘typical’ 18 x 18 – inch area -one
that represents the type of defects that you want to remove and starting with
the least abrasive polish and / pad combination and then proceed until the
defects are removed. This is the polish / pad combination you’ll need to use.
Selecting the correct pad / polish combination for the vehicles paint / defects
can take just as long as the paint correction process
Science of Scratches
Dictum [: a universally accepted
principle or rule]
Dictum I
In a clash between two
materials the harder material will come out unscathed – unscratched – and the
softer material will wear the battle scars.
Dictum II
Hard and soft are relative
terms; you can scratch the “hard” surface of your car’s paint finish with a
“soft” towel if I apply enough (localized) pressure.
Dictum III
This one is really short and
simple and rather obvious. Heat
makes solids softer; cold makes them harder. Keep that in mind
when detailing your car in the sun or on a really, really hot day.
OEM Paints
Glidden, Nason, Sherwin Williams, Sikkens, PPG Protective and Marine Coatings, DuPont™ Performance
Coatings, Spies Hecker® (part of
DuPont, used mostly by European auto manufacturers) and BASF
coatings, each of these companies have a range of several differing paints.
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). The clear coat
also has ultra violet protection added to protect the paint matrix.
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.
Nano technology Coatings
DuPont's SupraShield™, PPG's
Optech™ and CeramiClear™ Clear Coat are
all binary clear coat compositions using ceramic particles designed to offer
long-term scratch resistance, gloss and durability
Working with PPG, BASF and
Mercedes Benz, in 2003 Menzerna developed special polishes for use on PPG
CeramiClear™ Clear Coat for removing scratches, swirls and paint defects Beyond superior
abrasives, Menzerna has pioneered the development of polishes designed
specifically for the hard clear coats, like those used by General Motors on
the Corvette and PPG CeramiClear™ Clear Coat being used by Ferrari and
Mercedes- Benz.
Super Intensive and
Nanotechnology Polish are currently used by Mercedes - Benz in
Germany on their production line to remove swirls and over-spray incurred
during the painting process. Some OEM paint specification require a dense
(hard) clear coat paint- Audi, BMW, Nissan and VW and
others.
Relevant Publications
1.
Determining the Surface Hardness of Paint Coatings – Pencil
Testing vs. Instrumented Indentation Testing by Helmut Fisher
2.
Hardness testing of Organic Coatings - IUPAC
Current
Information
A little science is useful to
understand both the How and Why of detailing and to be of real
practical use, a subject like automotive detailing requires a great deal of
research, and updating as new products become available.
The advent of
materials like detailing clay, micro fibre technologies and finely milled micro
diminishing abrasives, suitable for ceramic nanotechnology paints are examples
of why it’s so important to monitor the industries new products, chemical
technologies and ideas that are constantly being introduced, as are the
techniques for applying them, hence all of the in-depth articles will be
up-dated and revised on a regular basis
Always be willing to learn; because the
more you learn, the more you’ll realize what you don’t know. You should never stop
learning, and your quest for information should be part of your everyday
process. It is
said that knowledge is power, with the caveat that it includes access to a
reliable information sources. I would like to think
that these articles become an asset to anyone who is new to detailing and to
professional’s alike, as well as industry experts who seek to advance their
knowledge.
I detailed my first vehicle at the age
of fourteen (1958) forty plus years later I started to write detailing articles
to share my experiences. For about fifteen years or so I started to contribute
to various detailing forums answering questions posted by neophyte’s,
enthusiasts and professionals alike. My mantra has always been
Experience Unshared Knowledge Wasted.
About the author - http://togwt1980.blogspot.co.uk/2015/05/about-author.html
This is not a product vendor’s
catalogue, nor am I a vendor pretending to be an educator, as there are a lot
of companies that are now sponsoring detailing forums, giving advice and
preaching that only the product they sell or manufacture are suitable. In
reality they are just advertisements, with the appearance of educators, mere
salesman. Those who have something to sell can be very persuasive, often using
marketing pseudo-science (i.e.
blurring the distinction between science and fiction) to make a great case
while completely ignoring meaningful facts, like their product adds nothing of
real value.
I purchase all the products I use, so
the endorsement is entirely personal and commercially unbiased, the product
recommendation is based on "Does exactly what it says on the tin" and
it suits my detailing goals. The products mentioned have been personally
subjected to extensive laboratory (using state of the art instruments and
methodologies in some of the world's most prestigious labs) as well as field
testing, and using the methodology and tools cited, which may or may not be the
same as those recommended by the manufacturer.
It has been my experience that they will
perform the task more than adequately, hence the personal recommendation, as
this testing is carried out without sponsorship I have no intention of
publishing any test results. Using the methodology and tools cited, which may
or may not be the same as those recommended by the manufacturer.
I
hope these articles are informative. They are based on the current status of
technical development as well as my experience with the products.
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
these articles as it helps other detailers further their knowledge.
As always if you have questions, I’ll do
my best to answer; bear in mind the only stupid questions is the one that was
unasked. Questions and/ or constructive comments are always appreciated
Copyright © 2002 - 2015 TOGWT®
(Established 1980) all rights reserved
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