> I don't know much about this but doesn't silver have a certain amount of
> "metal" in it that might affect radio performance when using internal
> antennas? I thought I read that a long time ago in a galaxy far far away.
>
> Troy
To block RF, it needs to be fully conductive, and they do indeed make
metallic paints for that purpose - e.g., 75dB RF attenuation and 40
milliohms resistance. But not any of several ordinary metallic paints
I tested (which may not contain any metal, but they don't say so on
the can, but one did show a few megohms of conductivity). Even with
some aluminum flakes, the worst potential effect is some refraction,
or phase shift due to "permitivity," unimportant for VHF comm and
nav. Maybe GPS, who just hates phase shifts, but couldn't get that to
happen. Confirmed also this A.M. with an aircraft antenna engineer.
Pasted below is a primer on metallic, pearl, and mica coats. One
point is getting it to come out right in amateur application isn't
always easy. Please forgive the length, edited as it, except the part
about about bird and bat guano! Best I've come across.
Regards,
Fred F.
FANCY TO FACTORY FINISHES
When using a metallic, you are no longer painting a color but are
applying a color effect. This effect is shown by the various shades
that are seen when viewed by the human eye. This broad spectrum of
color is the result of "reflective action"--metallic flakes do nothing
more than reflect a light beam that has penetrated the finish.
The following factors contribute to the varying look of metallics:
Stratification or the layering depth of these flakes determines the
amount of transparent color the light travels through, on the way in
and on the way out. Colors appear darker when the flake is closer to
the bottom and lighter when the flakes are near the top.
Flake size determines the quality and volume of light reflected.
Larger flake sizes reflect greater intensity of light--though not at
the volume of smaller flakes--and small flake sizes lighten the color.
Large flakes provide a glitter effect, but they don't always lighten
the color.
The biggest misunderstanding with metallic colors is what's called
"the edge effect." Metallic flake is made from a sheet of aluminum
that has been polished on both sides, and the pieces are generally of
the same thickness. Flake size is determined by the diameter of the
polished side. These sheets are then broken into specific grades or
sizes based on the dimension of the polished side. A small flake then
could have more surface area that is unpolished--the broken edge--than
that of the polished sides, resulting in a flake that will turn a
color more to the gray side than a reflective finish.
APPLYING METALLICS
Successful application of any metallic finish is the result of many
things coming together. But most importantly, painters must have a
mind-set for these finishes that is different from their approach for
all other coatings. Follow the rules.
Under-reduction or too fast a solvent has an adverse reaction in that
the material reaches the car without the proper amount of solvent,
preventing the material from flowing, leveling or stratifying. This
results in excess texture, off color and low gloss.
Spraygun selection, including tip size and flow rate, determines the
volume of material directed toward the vehicle and the atomization and
breakup of the fluid into tiny droplets. It is critical for the
painter to conduct a pattern and atomization check with every new
application. This check should be done on a test panel--not in the
open air. With the high-volume, low-pressure (HVLP) sprayguns
prevalent today, the air pressure, pattern and atomization must be
balanced. These guns will vary greatly with minor pressure
adjustments, as little as 2 or 3 psi.
Speed and distance are also crucial factors in matching metallic. The
finish must be applied with a 50 percent overlap while the gun is held
perpendicular to the surface at all times. The technician must also
maintain a constant, rhythmic speed.
Finally, flash time has an important role in the matching process.
Shorten the time between coats to darken the color. Applying the next
coat sooner re-wets the previous layer, which somewhat melts together
to let the metallic flakes settle more to the bottom. Extending the
flash or re-coat time does the opposite--each coat becomes a finish
unto itself. Excess flash time can create such problems as dull finish
or off-color side tones (flop), so follow the guideline of the paint
manufacturer for re-coat time.
An excellent guide to good matching is to make it your goal to place
the metallic flakes as close to the same position as the original
finish. Using options of speed, distance, reduction rate, flash time
and air pressure--singularly or in combination--the physics are:
Wetter equals darker, and dryer equals lighter. Do not change solvent
grades after the initial mix except to slow the evaporation rate.
A second technique that has worked for years is to always use one
grade slower solvent. This allows the stratification to occur,
increasing color development and minimizing reactive problems later.
It is much easier to lighten than to darken.
PEARLS, MICAS, and IRIDESCENT FINISHES
Pearl is the original iridescent additive that customizes a finish.
The base product comes from the interior lining of seashells (Mother
of Pearl) or Guanine crystal, the derivative of sea bird and bat
guano. When added to a translucent paint material, it alters the light
ray within the film and bends or refracts the ray, giving off a
secondary color. This action provides a completely new dimension of
color--a refractive effect.
The original pearls were platinum or gold in color and provided single
secondary color. Later, with further synthesis of the material,
various colors were added and the secondary color varied greatly. This
product served the industry well for many years, even though it had
some severe drawbacks. It would settle quickly when reduced, requiring
agitator cups or the dropping of ball bearings into the cup. Pearl
came in a heavy liquid form that settled like a paste in the jar and
made measuring next to impossible. Some products were in powder form,
and they didn't mix well, either. The products also had little
resistance to ultraviolet (UV) light, which weakened their refraction.
But the biggest problem started with the new paint chemistry in
urethanes--the pearl was not as homogenous or compatible.
The first true tri-coat result came from this--a base color, pearl and
clear. One of the drawbacks was the difficulty of repairs, again
because of the unpredictability of the material.
Those who say repairing these finishes is easy because there is no
evaluation point should try it and see how difficult it is. The
greatest difficulty with pearl is not being able to see what the
finished product will look like while you're applying the coating.
This is acceptable for the original applicator but not for the
repairer. For example, having an off-balance spray pattern results in
horrible streaks in the finish, but you can't see that until the
vehicle is taken outside. When refinishing with pearl, technicians
must take into account all of the standard factors that contribute to
a successful application, including mixing, distance, speed, air
pressure, gun selection, and spray pattern and atomization.
MICA
This relatively new product to the automotive industry has been around
for years, just in a different form. Cosmetic companies have been
using this organic rock for years to make lipstick, eye shadow and
other facial products.
Mica is taken from silica, a translucent rock in ample supply. It is
ground to a specific size and coated with titanium dioxide. This
coating provides a compatible surface for other paint chemicals and
also creates a silver or platinum color. Generally, the thickness of
the titanium dioxide determines the color of the mica, but additional
sizing and coating with other dyes creates the wide variety of colors
available today.
The first American vehicle to use this material was the Cadillac
Alante, and it was a disaster the first year with pearl white. Because
the base color and clear carrier were lacquer, the mica settled
differently--based on gravity--and required repairing an entire zone.
Some facilities used to refinish the entire car because they were
unable to produce an acceptable repair. The product changed the
following year, as did the repair procedure.
Harley Davidson, with the assistance of Ross Mattson and PPG
Industries, began providing true OEM finishes in the early '80s that
incorporated mica. The development of PPG Radiance inks and dyes came
---From this project. The door opened to this new glamour finish, and
production vehicles began using mica in the basecoat instead of as a
tri-coat.
Mica has some characteristics that set it apart from metallic and
pearl. Although refractive--it allows light to penetrate the particle,
bounce off the top surface or any of its internal surfaces--it is also
reflective, which is similar to metallic. This deviation of light rays
creates the mirro' effect seen when looked at from any angle except
"face up," or perpendicular.
Unlike pearl, mica is quite dependable in what effect it will provide.
But it is impossible to see or predict the outcome of the refinishing
job during application. Because it is a more stable material and
compatible with most chemicals, it is widely used today. The most
important aspect and difference is that the color of a mica finish is
determined by the number of coats.
[source: Automotive Body Repair News, www.abrn.com]
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