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By Jim Wilk, Licensed Auto Body
Technician
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This 2004 Mercedes Benz
ML320 plastic bumper cover had two damaged areas
I needed to repair. |
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The worst damage was below the fog lamp. To
repair this area, the bumper cover will have to
be removed. |
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The damage to the lower center area was minor. |
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I removed the bumper cover and
placed it on work table.
Removing the bumper was
time-consuming. Mercedes-Benz uses A LOT of fasteners. There
was at least a dozen bolts holding this trim piece to the
car.
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crack was aligned back together, I cleaned both the
front and back surfaces with lacquer thinner. Then
I applied masking tape over the crack on the
back side. This is important. Masking
tape on the back side helps to prevent the glue
from dripping through the crack.
Using a router tip
mounted on a drill (red arrow), I carved a groove
along the crack, taking care not to cut
completely through.
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This bumper cover has a reinforcement formed on
the back side. |
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On bumper covers that are open on the
back side, I would tape the front side of the
crack together. Then using 80 grit sandpaper, I
rough up the back surface. Next I would slather
some glue to the back surface, and embed a patch of metal door screen or
Kevlar joint tape in the glue. Then I would apply another
layer of glue to hold it all together and let it dry
overnight. |
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| On the center
crack I cleaned both sides, pushed the mis-aligned
plastic back in place and taped the back side. |
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I carved a groove along the crack with the router
tool, being careful not to penetrate through. |
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| With a pneumatic
sander and 150 grit sandpaper, I sanded a
large area around the grove. (80 grit
sandpaper is too rough for the visible side of
the cover.)
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These areas were ready for the plastic
bumper epoxy glue. |
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| Using lacquer
thinner I cleaned the area again. I found that cleaning the area a minute or
two before applying the glue softens the plastic and promotes better adhesion.
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Using latex gloves is necessary.
This glue can penetrate your skin and leave a
residue for a week. There is a special glue
gun and tip used to apply this 3M glue and other
glues.
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| On these
plastic covers only certain
glues work and hold up
under road conditions... 3M, Rubber Seal
and Sems are the most commonly used products in
the auto body trade. |
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With the gun and applicator tip, I laid down a heavy
bead in the groove. |
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| Using a
plastic spreader I pushed the epoxy material in
the groove and smoothed it out. |
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After applying the glue, I let it dry for at
least 8 hours. When I removed the tape, the back
side looked like a nicely rounded weld bead following the
crack. |
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| The longer
these epoxy materials dry the easier they are to
sand. |
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When sanding, patience is a major factor. I
started sanding with
150 grit sandpaper, pressing lightly and
not staying in one area too long. |
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| If they get
too hot these epoxy
glues will gum up the sand paper and start
separating from the plastic. |
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Fiber abrasive discs can also be used for rough
sanding of these epoxy glues... but you need to
sand lightly. |
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| This fiber
disc is a 3m brand. These discs have a threaded
stem that screws into the arbor, which is mounted to a
die grinder. There is also an arbor
that mounts to a drill or angle grinder. |
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I sanded the patch with 150 grit and then 240 grit
sandpaper.
This prepared the surface for a coat of
flexible putty. |
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| Then the area
was smooth and the contour was established. |
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The crack in the center of the bumper cover was ready for a thin layer
of flexible putty. |
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| The area
around the fog lamp had a few crevasses,
probably from air bubbles that got into the
epoxy glue. The tiny black spot to the right of
the round sanding wheel is one such trouble
spot. |
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Using a small sanding wheel I carved a beveled
edge on the air bubble so the putty would adhere
properly. |
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| I applied the
flexible putty to the repair areas. |
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The putty I used will adhere to a painted
surface if it is sanded. |
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| Using a
sanding block I hand-sanded the flexible putty,
first with 150 grit, then 240 grit, and finally
320 grit sandpaper. |
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This putty sands easily with very light pressure. If
you push too hard it will cause
waves in the putty and won't blend in with the
paint line. After sanding, the bumper cover
was ready to be primed and painted. But first,
the dust needed to be removed with compressed
air. |
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| Next it was
primed with Matrix MP-2K urethane sandable
primer. |
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Then I lightly block-sanded the primed area with 320
grit sandpaper to smooth it out. I was careful not to
sand through the primer. |
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| The bumper
cover was painted with Matrix System
base coat and clearcoat and allowed to dry
overnight. The next day I mounted the cover on the vehicle. |
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The finished repair job... it looks as good as
new. The cost to replace this bumper cover is
almost $800... which explains why they repaired
it instead of replacing it. |
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This project was performed at
R-TECH Auto Body,
in Traverse City, Michigan.
More Info:
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Tools Used:
- Basic Mechanic's Tools
- Dual Action
Sander
- Drill with Router
Bit
- Putty Spreader
- Sanding Block
- Small Sanding Drum
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Materials Used:
- 3M #4240 Semi-Rigid
Plastic Repair Adhesive
- Sandpaper Discs:
150, 240, 320 grit.
- Rubber Gloves
- Flexible Auto
Body Putty
- Primer and Paint
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