Understanding Undercoats
Undercoats create the foundation for quality refinish work. In many ways, they recreate the OEM treatment process.
By Michael Regan
9/1/1998
The understanding and proper use of primers
in the paint shop not only contribute to customer satisfaction
but to the self-satisfaction of having done your job right. Undercoats
create the foundation for quality refinish work.
Vehicle owners are very aware they like how
a paint job looks on their cars, but they may not know that a
great deal of preparation and knowledge about the protective aspects
of refinishing are at work beneath the surface of their "baby."
Primers and other undercoats figure heavily into this protective
equation. Let's have a look.
On the OEM End
The vehicle manufacturers have gone through
several generations of methods to prepare vehicles for painting.
Restrictions on emissions; consumer demand for a durable, long-lasting
return on their investment; and advances in chemistry have created
state-of-the-art finishes. Matching the OEM appearance on these
vehicles requires a unique set of skills, and matching the OEM
protection requires an understanding of what was done to the vehicles
before topcoats were ever applied.
The OEM process works something like this:
After the basic body shell is constructed and all welding and
pre-assembly are done, the entire body is solvent washed to remove
oil, grease and other contaminants. It's then blown and/or vacuumed
to remove filings and dirt particles and typically cleaned again
and then rinsed. This incredibly thorough cleaning is followed
by a trip through a phosphate-treatment bath, which creates a
bonding layer on the metal surface for the actual primer to adhere
to. The now-treated body is then dried and moved on to the Electrodeposition-coatings
(E-coat) line.
E-coats have been the OEM mainstay for many
years. Originally conceived to coat the interior of food and drink
cans, the technology has been widely adapted by manufacturers
due to several process-related advantages. Transfer efficiency
is very high - 95 percent or better. Because the object - in our
case, the vehicle body - is fully submerged into the E-coat tank,
hidden and hard-to-reach areas emerge with full protection. Also,
the film thickness of the primer using this process is very even.
For the manufacturer, E-coat is a big win. Quality is high, emissions
are low and there's a huge reduction in waste cost versus a spraying
operation. The E-coated surface can also continue through additional
painting operations without being sanded.
After E-coat and prior to topcoating, some
vehicle manufacturers apply chip-resistant undercoats to hood,
roof and rocker areas. For some vehicles, primer-surfacers are
applied to remove imperfections in metal surfaces that the E-coat
can't fill. After all priming and primer-surfacer operations are
complete, the vehicle moves on to the regular paint line for final
topcoats.
Now It's Your Turn
At no time in recorded history has a professional
refinish technician been confronted with more choices as to which
materials to use on a vehicle than today. And, in some cases,
it depends on what part of the country you're refinishing in.
What's the best approach to sort through all
these offerings and to come up with the right choice for your
situation? Your answers to the following questions should help
you decide.
1. What's our shop trying to accomplish?
Are you doing late-model collision work? Are
you trying to restore the vehicle to pre-accident condition? Does
your shop offer a lifetime guarantee on refinishing work? Is the
owner of your shop concerned about lasting quality? Do you take
pride in being the best at what you do?
If you answered yes to the above questions,
my guess is that you're already committed to a refinish system
and that you've most likely been trained in the proper choices
of materials and methods to restore protection to the vehicles
you work on.
2. What does our shop environment allow me
to use?
A modern refinish shop with adequate spraying
space, good ventilation and convection or infrared baking capability
allows you to use the latest and greatest two-component undercoats.
With the proper equipment and professional scheduling by the front
office, you can apply a complete system that meets or even exceeds
the vehicle manufacturers specifications - and this can be accomplished
in a cost-effective, highly productive fashion.
If your paint shop situation is currently
less than ideal, you'll have to spend a little more time choosing
products and giving some thought to dry times and overspray on
other vehicles in the shop. You should also consider the respirator
safety of any employee in the area you work in, including yourself.
3. What constitutes the best system for me?
Your paint distributor is one of your best
resources for up-to-date information on products and methods that
will be most effective for your operation. If you're unsure that
you're using the best system available, arrange a meeting with
your distributor and ask him to schedule the paint company rep
to sit in. The subjects discussed should include each step of
the process your shop follows once the vehicle hits the paint
shop.
Picking a Primer System
The chemistry available to you, starting with
bare metal, has come a long way. Acid-etching bare-metal primers,
urethane primer-surfacers, and tintable primers and sealers all
contribute to an amazing arsenal of modern tools. If you remember
the days of metal cleaning/rinse/conversion coating/rinse/apply
product, you probably realize that today you're in heaven.
When you get a new OEM panel today with E-coat,
there are several choices of primer-sealers that you can apply
after you chemically wash the new panel. You don't have to scuff
it! And remember how one of the process advantages of E-coat to
the manufacturer is that it can continue through to other painting
operations without being sanded? This remains true for refinishers
too. (Check with your paint supplier for specific product recommendations
regarding this.)
Acid or self-etching primers replace the cleaning
and phosphate-conversion process used by the manufacturer; this
step creates long-term corrosion protection and prepares the bare
metal for maximum adhesion of subsequent product.
Two-component primer-surfacers are the next
step and provide filling and leveling to repaired areas. Most
of the paint companies offer these in isocyanate-containing urethane
versions, as well as non-iso/low-VOC products. The advantage of
the 2K products is that they chemically crosslink and cure to
a non-reversible state. The old lacquer and acrylic lacquer primer-surfacers
dried but never cured; they could always be reversed to a liquid
state by the addition of solvent and/or heat (always meaning forever).
The disadvantages are obvious. (We used to think that if we primed
a car and let it sit in the sun for a few days, it would really
get that primer hard.) The new products dry, cure and remain stable
until oblivion. They create a marvelous substrate for topcoat
application. There's virtually no sand-scratch swelling, shrinkage
or dieback caused by solvent absorption.
Tintable primer-surfacers and sealers also
are gaining popularity. The increased use by manufacturers of
tinted undercoats, which are less costly than topcoats, has created
the need for these products. They can also be quite effective
as a non-intrusive color indicator in the event of a stone chip
or scratch on a vehicle. It would be nice, therefore, if the color
information would be utilized to allow us to match these colors
with a primer or sealer. Most current information from paint companies
that includes specific underhood colors does so only in topcoat
qualities. To match the tinted undercoat on a vehicle, the painter
adds toners from his mixing machine to the primer mix, along with
a catalyst and reducer, and then applies this now-tinted mixture
to the vehicle. In some cases, this will be the final coat in
a jamb or the inside of a hood or decklid.
As for sealers, they can be used for a number
of reasons. They don't offer the filling qualities of a surfacer,
but when painting new, undamaged panels, they provide the intercoat
adhesion needed for topcoats to adhere. The 2K sealers offer the
chemical stability needed for high-quality topcoat appearance
and long-term color holdout. Depending on the topcoat system your
shop uses, an application of sealer can generally take the place
of one sanding step of the primer-surfacer. For example, if you
normally wet sand with 600 grit prior to topcoat, you could go
down to 500 grit wet, which cuts faster, and then apply a coat
of sealer prior to topcoat. The use of a sealer isn't absolutely
necessary if you're using other 2K undercoats. However, the choice
to use them could be affected by your shop situation.
Understanding Undercoats
Undercoats are, indeed, the foundation for
quality refinish work. They protect the metal or other substrate,
create a smooth surface for topcoats and provide lasting quality
to the vehicle. In many ways, they recreate the OEM treatment process.
The materials today are far superior to those
available even a few years ago. The biggest improvements have
been driven, in large part, by environmental dictates. The chemists
who design these products have responded to the challenge by creating
the most user-friendly products ever. They're the most stable,
durable undercoats ever devised.
No matter what needs must be met to accommodate
your environment and work standards, products are available to
help you achieve your goals. If you think you've been left out
of the loop, talk to your paint supplier about his latest offerings.
You might be surprised at what the most current generation of
undercoat products can add to your bottom line. And there's still
nothing quite like a satisfied customer - which is exactly what
smart technicians producing quality refinish work create.
Writer Michael Regan is president of The
J.J.R. Company in Cleveland, Ohio, and a contributing editor to
BodyShop Business.