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By Jim Wilk, Licensed Auto Body
Technician,
And Bruce W. Maki, Editor.
Start:
If you live in the northern United
States or Canada, you know how road salt can make cars rust
quickly. Some people like to apply undercoating to the
bottom body surfaces of a new car, before the road salt has
had a chance to begin the corrosion process.
Those of us that work in the auto
body business have seen many cars that essentially rust from
the bottom up. Since road salt is often mixed with sand,
there are a lot of abrasive particles on the road that get
splashed onto the bottom of cars, causing nicks in the
factory coating. Any nicks or scratches in the metal coating
can rust, and salt accelerates the corrosion process. If
steel is kept completely covered with a non-porous coating,
it won't rust... but it's difficult to keep that coating
perfect.
While undercoating a used car is not
usually recommended, a customer who had purchased this 2003
Jeep Cherokee in the desert area of Arizona asked us to
apply undercoating to the vehicle to keep the bottom sheet
metal from rusting in our Northern Michigan winters.
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Before applying
undercoating, the surfaces must be clean and
dry.
The day before, we
lifted the car on our shop hoist and
power-washed the underside. Then we let it dry
overnight.
Then we hoisted the car
again to apply the undercoating. |
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Of course, the average
do-it-yourselfer probably doesn't have a hoist in their
garage. But, some cities have businesses that rent hoists by
the hour.
It's possible to undercoat a vehicle
that is raised up on jack stands. You don't have as much
room to maneuver as the hoist, but it's been done.
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The red arrows point to
some of the not-so-obvious spots on the bottom
of the car that need to be undercoated. |
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Some of these areas are hard to reach because
they are behind the exhaust system or drivetrain
parts. |
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Other areas are more obvious, such as the
C-shaped frame channels. |
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| These brake
lines and fuel lines would be rusted after just
one winter up north, but this 7-year-old car had
virtually no rust on the metal lines. |
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It can be tricky to spray undercoating behind
the suspension control arms. The control arms
themselves may not need undercoating if they are
made from heavy steel forgings, but these
control arms are made from steel stampings, so
we coated them. |
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| We used 10
spray cans of 3M brand rubberized undercoating
on this car. We ended up applying 3 coats
before all the cans were used up.
We started spraying at the front of the
car... |
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...and worked towards the back. We didn't
remove the wheels because the rear wheel wells
had undercoating applied at the factory, and the
front wheel wells have plastic shrouds above the
tires.
On some cars it might be helpful to remove
the wheels, especially if the car was only
placed on jack stands. Then you would have more
elbow room to work. |
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Note:
Follow the
instructions on the spray can. The
instructions should describe how much the
can needs to be shaken, how far to keep the
spray nozzle from the car, and how long the
material takes to dry. |
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| After
applying the first coat of undercoating there
were still some spots that didn't receive a
thorough coat. After the first coat had dried
for about 15-20 minutes, we applied a second
thin coat. |
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Warning:
Undercoating
materials are often made from petroleum
products, so they can be flammable and/or
harmful to your health. Use only in a
well-ventilated area. |
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We did this job with two people... one held a
flashlight and looked for thin spots and bare
spots while the other used the spray can. It
can be hard to find some of the thin spots
because you are looking for a not-so-black area
amongst all the black areas. |
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| We avoided
spraying undercoating on the heat shields (red
arrow) above the exhaust system. |
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After the second coat there were some thin areas
that showed up as the liquid dried. |
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| We sprayed
the underside of the body around the hump above
the rear axle. We also sprayed the suspension
control arms and any metal brake lines. (The
red arrow indicated the back tire, the yellow
arrow points to the rear axle.)
We also applied undercoating to the areas
where the shock absorbers mount to the car body.
If these areas rust, it can be expensive to
repair. |
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This 7-year-old car looks quite new, so it's
worth spending a few bucks to prevent rust on
the underside of the body. |
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More Info:
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Tools Used:
- Power Washer
- Shop Hoist
- Flashlight
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Materials Used:
- Rubberized
Undercoating, 10 Spray Cans
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