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How to repair plastic bumper covers.
Learn tips on how to repair and bond plastic bumpers.

I have been asked many times how to repair a plastic bumper cover. So I made this short article and kept it simple without all the fancy long plastic names that manufacturers like to use.

I had the opportunity to meet Dick Jacobs, he was the founder of Duramix, the plastic adhesive repair company which was bought out by 3M a few years back.

Dick was one of us, a very simple focused man, constantly thinking about what innovation he could come up with for the Duramix product line and spoke in very simple terms but extremely thorough.

One day we picked him up at the airport, we got in the car and I asked him a technical question. We drove in heavy traffic for about 45 minutes, had dinner for about 2 hours and then drove him to the hotel which was he was staying at. He got off the car and wrapped up the answer to my question about micro-spheres in SMC panels.

Everyone in the car that day got a very educational crash course about polymers, plastics, micro-spheres and SMC composition. The other lesson is never ask a technical question to an engineer unless you have a lot of time and note pads.

I am going to show you a simple method of repairing plastic panels and TPO bumpers.

1. There are only four types of plastic you should know about in the automotive industry.

  1. TPO - Sands in chunks, melts like butter with high speed grinders, feels like wax and gets stringy when hot.
  2. Thermo-Set - Flexible or Rigid, sands powdery, does not melt with high speed grinders and easy to repair.
  3. SMC- Sheet molded Compound - New Corvettes have it and looks like fiberglass with white powder. We will cover SMC in another article.
  4. Poly propylene - RV Water Tanks, Dirt Bike Fenders - Not recommended for repairing or painting.

2. Any Plastic Repair system you choose will contain these components.

  1. A cleaning agent
  2. Adhesion Promoter
  3. A mixing gun applicator
  4. Adhesive/Material for the structure repair
  5. Mixing Tips
  6. Flexible contouring plastic and backing re-enforcement material
  7. Finishing Past/Putty

3. Lets talk about TPO Plastic. This is the most difficult plastic in the market to repair. It contains 3% to 5% wax based mold release within the material. So that's the problem, how do we get adhesive or paint to stick to mold release? Many shop owners and technicians today will just rather not touch it and throw the bumper away. TPO is repairable, but you have to take two precautions without exception. If you can fix TPO bumpers, you can fix the thermo set the same way, if you are not sure which of the two you have on hand, treat it like TPO. If you don't know what TPO stands for, it does not matter because with my system you don't have to know. Repair it like this and you will be fine.

  1. After you sand the repair area follow these cleaning steps, they are the key in repairing plastic bumpers successfully. Use a gray or white scuff pad and scrub the repair area of the bumper or plastic panel with the cleaning agent first, a simple wipe is not going to work. It may work sometimes but may fail in others. Neon Bumpers may require two or three cleaning steps. The main reason bumper repairs fail is by not doing this step thoroughly. How do you know the plastic is clean? I use the water test. If the water beads up like a freshly waxed car or just disappears off the plastic, clean it again, then try the process again until the water stays there wet for a while.

    Do not use sanding paste on raw plastic. Sanding paste is not part of the plastic repair system mentioned above nor solvent based wax and grease removers. These products will contaminate the plastic. You will have mold release freshly activated by wax and grease remover solvents. That's not good. Always use the cleaning agent the manufacturer recommends.

     

  2. For this example I am going to use a bumper that has a 9" x 1/4" ripped hole shaped like a letter " L "  right in the middle of the  corner. This seems to be a very common repair in the auto body industry.
    1. I use a small angle air grinder with a 2" 36 grit disc to sand off the rugged edges of the damage until it is all smooth and shaped like a bowl or a river. This way the structure adhesive has more surface area to grab. The edges flow downward at an angle for 1/2". Some people refer to it as  "bevel sand the edges" . Do not use the V groove method because this is too narrow for the adhesive to grab. Picture the V groove but spread out over a good area like a bowl. 
    2. Optional Step. Drill 1/8" holes in the surface area grooves about 1" apart, This will allow structural material to flow through the holes and make an H re-enforced repair with the holes acting as rebar fillings.
    3. Back sand the front and back with 80 grit sand paper.
  3. Clean the repair again, once the panel is clean, you have 30 minutes to apply your materials. Do not wait longer than that because the mold release will start to emerge to the surface of the bumper. The faster you get on it after the cleaning the better your repair will be.
  4. Apply Adhesion Promoter on both sides and wait at least 10 minutes or what ever the manufacturer recommends.
  5. Purge the two part cartridge before placing the mixing tip on. After placing the tip, purge about 2 inches of material on a cardboard and make sure the material looks all one color.
  6. Apply structural repair material to a backing re-enforcement piece, enough to cover at least 1 inch around the back side of the repair. Apply the backing material piece to the back side of the bumper and the let the material ooze out through the holes and groove, then gun some more material while it is still wet and place a contouring plastic on the top and spread it out using a spreader or your hand to match the contour of the bumper edges.
  7. Wait one hour, then sand with low RPM speed.
  8.  Finish off with a Plastic Finishing Putty or filler.
     

This guide is generic in content and we recommend you follow the manufacturers instructions. Each manufacturer may contain different recommendations in order for their product to perform the best.

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