Projects & Repairs --> Collision --> Repairing Trim Plastic --> 2004 Mercedes

Car with crack in plastic bumper cover.

Repairing The Plastic
Bumper Cover On A
2004 Mercedes Benz ML320

Fixing Flexible Urethane Parts With Epoxy Adhesives.

In This Article:

The cracked plastic bumper cover is removed and the cracks are opened wider with a router. Epoxy adhesive is applied, allowed to cure, sanded smooth and painted.

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Skill Level: 4-5 (Advanced to Professional)

Time Taken: About 4 Hours (Over 2 Days)

By Jim Wilk, Licensed Auto Body Technician



This 2004 Mercedes Benz ML320 plastic bumper cover had two damaged areas I needed to repair. 

Mercedes ML320 with cracks in plastic cover around front bumper. 


The worst damage was below the fog lamp. To repair this area, the bumper cover will have to be removed. Close-up of crack in plastic bumper trim. 


 Minor crack in lower part of front plastic bumper cover. The damage to the lower center area was minor.

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.


After the 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.

Cutting a groove along crack to prepare for fixing. 


  This bumper cover has a reinforcement formed on the back side.


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.


On the center crack I cleaned both sides, pushed the mis-aligned plastic back in place and taped the back side. Pushing cracked plastic back into alignment. 


 Carving groove around area of crack in plastic automotive bumper trim. I carved a groove along the crack with the router tool, being careful not to penetrate through.


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.)

Sanding area around crack with power sander. 


 Cracked plastic bumper trim cover ready for adhesive. These areas were ready for the plastic bumper epoxy glue.


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.  

 Cracked plastic after being cleaned.


 Epoxy glue and dispenser tool, for fixing cracked automotive plastic. 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.


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. 3M 4240 plastic repair adhesive. 


 Applying adhesive to crack in car bumper cover. With the gun and applicator tip, I laid down a heavy bead in the groove.


Using a plastic spreader I pushed the epoxy material in the groove and smoothed it out.  Spreading epoxy plastic adhesive with flexible spreader tool. 


 Glue after drying for 8 hours. 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.


The longer these epoxy materials dry the easier they are to sand.  


 Sanding the plastic adhesive after curing. When sanding, patience is a major factor. I started sanding with 150 grit sandpaper, pressing lightly and not staying in one area too long.


If they get too hot these epoxy glues will gum up the sand paper and start separating from the plastic. Sanding the plastic adhesive being careful not to overheat it. 


Fiber abrasive disc for rough sanding plastic repair patch.  Fiber abrasive discs can also be used for rough sanding of these epoxy glues... but you need to sand lightly.


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.

Roloc disc 


 Sanding repair patch with 150 and 240 grit sandpaper. I sanded the patch with 150 grit and then 240 grit sandpaper.

This prepared the surface for a coat of flexible putty.


Then the area was smooth and the contour was established. Smoothly feathered repair patch. 


  The crack in the center of the bumper cover was ready for a thin layer of flexible putty.


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. Plastic repair technique.


 Grinding plastic with drum sander. Using a small sanding wheel I carved a beveled edge on the air bubble so the putty would adhere properly.


I applied the flexible putty to the repair areas. Flexible auto body putty applied to plastic bumper. 


 Body putty will stick to painted surfaces if paint has been sanded. The putty I used will adhere to a painted surface if it is sanded.


Using a sanding block I hand-sanded the flexible putty, first with 150 grit, then 240 grit, and finally 320 grit sandpaper. Sanding body putty by hand. 


  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.


Next it was primed with Matrix MP-2K urethane sandable primer. Bumper cover after being painted with primer. 


  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.


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.

Plastic bumper after repairs, crack has been fixed. 
 Mercedes after fixing cracked bumper.  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.

This project was performed at R-TECH Auto Body, in Traverse City, Michigan.


More Info:

Tools Used:

  • Basic Mechanic's Tools
  • Dual Action Sander
  • Drill with Router Bit
  • Putty Spreader
  • Sanding Block
  • Small Sanding Drum

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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Written November 6, 2008