How can the car manufacturers do this? Haven’t
they ever been in the Midwest during the winter? Out here, we
salt the salt to make sure we melt the ice. Or how about the cars
on the sea coasts, where morning dew brings a daily salt bath.
The OEMs can offer these 100,000-mile rust-through
warranties, in part, because they’ve doubled or tripled the amount
of seam sealer (which is designed to keep moisture out of areas
it doesn’t belong) and used epoxy (or other) bare-metal primers
on all bare metal, along with some form of galvanizing.
So why the OEM concern with rust? Because
corrosion was universally identified as a high consumer priority
when car manufacturers polled potential new-car buyers. Everybody
they asked said having the car body still intact five years later
What’s important to you, however, is that
you take the proper steps to restore corrosion protection when
performing repairs. The vehicle owners’ contracts with their insurance
carriers call for restoring the car to its preaccident condition
– and this certainly includes the 100,000-mile rust-through warranty.
If you fix one of these cars and rust eats through the repair,
your shop will be held responsible, period.
Also keep in mind that the actual repair can
do additional damage to the built-in corrosion safeguards. Heating
galvanized metal will burn off the zinc coating, pulling damaged
panels pulls apart seam sealers and cutting holes of any kind
in late-model vehicles gives rust a nice head start.
OEMs vs. Rust
To help battle rust, the OEMs utilize the
galvanization process, seam sealers and bare-metal primers:
- Galvanizing – As I’m sure you know, this process bonds
a layer of zinc onto steel. Ordinary bare steel will allow the
formation of iron oxide, which is a red, scaly compound formed
by the combination of iron, water and oxygen. The metal zinc,
on the other hand, will combine with water and oxygen to form
zinc oxide – a white-colored compound that remains attached firmly
to the base metal, unlike iron oxide that scales up away from
the metal. By bonding a layer of zinc to the steel, the stage
is set for "sacrificial corrosion" – the magic of galvanizing.
Picture your galvanized garbage can. If you take an ice pick and
make several long scratches down the sides of it, sacrificial
corrosion will cause the scratch to "heal" before rust
can begin. What happens is the moisture and the oxygen will combine
with the zinc before they can combine with the iron in the steel.
So before red rust (iron oxide) can form, zinc oxide forms instead.
Zinc oxide creeps across the ice-pick scratch and meets the zinc
oxide creeping across from the other side. Before the iron can
be exposed to moisture and oxygen in the air, zinc oxide covers
over the scratch.
What happens, you ask, when you scratch the same spot a second
time? Red rust will result. The layer of zinc oxide that formed
over the original scratch is much thinner than the original layer
of zinc, and not enough remains to do the same trick a second
How the car manufacturer bonds the zinc and the steel together
is slightly different for different brands. Each OEM figures it
has the best solution to this process, and each special process
and each secret ingredient is designed to get the zinc to corrode
before the steel can.
The following metal treatments are used by assorted vehicle manufacturers:
one-side coated, two- side coated, hot-dip galvanize, hot-dip
zinc iron, electrogalvanized, electrodeposited zinc nickel, zinc
manganese electroplated and zinc vapor deposition coated.
Coating the steel with zinc takes the steel out of the iron +
water + air equation, while most other rust-prevention methods
try to take either the water or the oxygen out of the equation.
- Seam sealers – These products try to seal out the air,
the water or both, and they’re also used to keep areas of the
vehicle free from rust. Depending on the application, seam sealers
need great flexibility, great impact resistance or great adhesive
Again, each vehicle manufacturer has its own solution to these
problems. Some vehicles seem to be covered with brushed-on seam
sealers, and still others have sealers between the panels that
don’t want to come apart, even when you want them to.
- Bare-metal primers – A great potential for rust exists
when a rock chips off the topcoat, exposing the bare metal. No
finish can resist rock chips for long, and the hardest finish
there is can still be removed by blasting rocks at it long enough.
- Epoxy (or other bare-metal) primers prevent the moisture
from creeping under the finish. If a flying rock chips off the
paint and exposes the bare metal, conditions would become favorable
for the formation of iron oxide – except that the moisture-tight
primer prevents the rust from crawling under the finish.
Restoring Corrosion Protection
I-CAR has an excellent program for restoring corrosion protection
to collision-damaged cars. If you haven’t enrolled yet, do so
In its abbreviated version, here’s what the I-CAR course covers:
The different OEM corrosion-protection measures all read about
the same. The difference between your shop and the car manufacturer
is that the OEM facility has brand-new clean metal, high heat
and the help of electrically charging the part or the primer.
While the choices out here in air-dry refinish land aren’t nearly
so controlled, great results are still possible.
The first step, as always, is to make sure the metal to be painted
is clean. Wax and grease removers do the best job getting contaminants
off the panels, and a mild acid mixed with water will clean the
bare metal of any remaining contaminants.
This first-step metal treatment must be rinsed off with clear
water to prevent a dry crystalline film from forming on the metal.
If the mild acid evaporates before rinsing can occur, rewet with
the acid cleaner before rinsing.
A second-step conversion coating is designed to set up a chemical
reaction on the surface of the metal to promote adhesion of the
primer. Both a first-step cleaner and a second-step conversion
are available for most types of metal found on the car.
The system for bare or galvanized steel isn’t the same as the
system for aluminum or magnesium, so make sure you use the right
products for the job. In some cases, self-etching primer, which
contains the acid right in the primer, can be substituted for
the two-step treatment. Ask your paint rep or jobber for advice.
Generally, sandblasted metal shouldn’t be metal etched because
it’s hard to get the acid out of the pits.
I-CAR recommends a separate primer be applied before using primer-surfacer,
a necessary step to seal the weather away from the substrate.
Whether self-etch or epoxy, these primers bond to the substrate
like nothing else, forming a moisture barrier that keeps water
away from the metal.
These primers generally are not designed to be sanded. A primer-surfacer
contains talc (hard clay) to act as fill – useful in bringing
the repair up to contour. The talc fills the scratches, causes
most of the dust in the air when sanding and can act like a wick.
Talc also absorbs moisture. (If water didn’t pass through clay,
your front yard would be under water every time it rained!) Applying
a primer-surfacer containing talc directly to bare metal
generally won’t provide the same results as using a separate moisture-tight
primer because the talc will allow water to pass through the primer-surfacer.
When replacing panels, most vehicle manufacturers and I-CAR call
for using a weld-through primer, which is basically 98 percent
zinc in an alkyd resin solution. The zinc sandwiched between the
two steel panels will help prevent rust from forming at the panel
Most directions call for carefully removing the excess weld-through
primer that remains on the panel because the paint manufacturer
doesn’t want you to build a high-tech finish on a low-tech alkyd
resin base. Likewise, punching holes in the metal panel invites
in moisture and oxygen where they can find some iron and begin
to corrode. I-CAR says the area of the panel with a drilled hole
is nine times more likely to rust. This helps explain the popularity
of pin welders – no hole is created when repairing collision damage.
Fast Isn’t Best
Like so many processes in collision repair, the quick solution
isn’t necessarily the best. Cutting corners during repair often
means skipping the steps necessary to guarantee the customer’s
vehicle is properly protected against rust.
If you can’t get paid to use weld-through primer or inner-panel
rust proofing, don’t offer a long-life warranty. Remember, without
a conscientious effort on your part to replace all the steps the
car manufacturer took to prevent rust, your repair won’t last
as long as the rest of the
Writer Mark Clark, owner of Clark Supply in Waterloo, Iowa, is
a contributing editor to BodyShop Business.
Check It Out
When restoring corrosion protection, you need to remember:
- Make sure the metal to be painted is clean. Wax and grease
removers will get contaminants off the panels, and a mild acid
mixed with water will clean the bare metal of remaining contaminants.
Rinse this first-step metal treatment with clear water.
- A second-step conversion coating is designed to set up a chemical
reaction on the surface of the metal to promote adhesion of the
- In some cases, self-etching primer, which contains the acid,
can be substituted for the above two-step treatment.
- I-CAR recommends a separate primer be applied before using
primer-surfacer, a necessary step to seal the weather away from
- Applying a primer-surfacer containing talc directly to bare
metal generally won’t provide the same results as using a separate
moisture-tight primer because the talc will allow water to pass
through the primer-surfacer.
- Use a weld-through primer when replacing panels, which will
help prevent rust from forming at the panel seam.
- Remove the excess weld-through primer that remains on the
panel because the paint manufacturer doesn’t want you to build
a high-tech finish on a low-tech, alkyd resin base.
- Punching holes in the metal panel invites in moisture and
oxygen where they can find some iron and begin to corrode. For
this reason, pin welders are gaining in popularity because no
hole is created when repairing collision damage.