Repair Cracks and Tears on your Bumper Cover for Less Than your Deductible

By January 14, 2015Bodywork
How to Repair a Cracked Bumper

Although they can’t compare to the beauty and shine of the chrome bumpers that adorned cars up until the late 80s, plastic bumper covers are actually good for us DIYers. They are easy to fix if they get scratched, cracked or even torn so you can save your deductible for more important things – like bacon and swap meet treasures.

So toss out all those bumper stickers featuring witty banter you were planning to use and don’t even think about going for that can of el-cheapo bondo and matching spray paint! All you have to do is apply skills you already have like grinding, sanding, sculpting and painting to working with plastics.

TOOLS

The repair kit brand isn’t as important as sticking with that same brand for all the products throughout the repair so you don’t have compatibility issues. Double check that the kit works with your plastic and if you’re not sure, get a kit for TPO or thermoplastics.

1. Remove the Plastic Bumper Cover

For a small crack or scratch in the paint I know you might be tempted to just leave the bumper on the car but removing it allows you to work at a better angle and more comfortably on a table or saw horse, protects the surrounding panels from damage during the repair, and saves you time by not having to mask the front of your car when it comes time for paint.

Today’s bumper covers are held on by a variety of hidden fasteners, self-locking tabs, screws and bolts. Depending on the manufacturer, you may see variations in style and location so you’ll have to do a little hunting to find all the fastening points.

There are typically a combination of round plastic fasteners, bolts and screws under the hood or trunk (depending on whether you’re working on a front or rear bumper), along the wheel wells and underneath the car. Once all this is removed you should be able to pull off the bumper cover with a few tugs. If it feels stuck, look for additional fasteners you may have overlooked or a push-tab that is still engaged.

2. Identify Your Plastic to Select the Correct Adhesive

Not all bumper covers are made from the same type of plastic and the repair kits you’ll see in the market are specific to certain compounds. Flip the bumper to its backside and look for one of these stamps:

  • PP, PPO, TPE, TEO or TPO (thermoplastics) – These plastics make up about 80% of bumpers because they’re better suited to large volume runs. The excess that results from the molding process can be recycled back which makes them environmentally friendly and cost-efficient. But the molding process also introduces a small percentage of wax-based mold release into the plastic which repels paint and adhesives so the surface requires an extra step while prepping. The surface smears and melts easily with high-speed grinders and sanders and gets stringy when hot so you may have to let the repair area cool from time to time while sanding.
  • PUR or TPUR (thermosets) – These plastics make up another 15% of bumpers and are more suited to low-volume and large parts than high production runs. Tooling costs are higher and trim waste can’t be recycled so they’re usually landfilled. The surface powders when ground or sanded so they’re easy to repair.

Once you identify your plastic, you can whip up the perfect recipe of products or ready-to-use kit for your repair. If you’re still not sure what plastic you have, treat it like TPO as we’re going to do here and you’ll have all your bases covered.

3. Prep the Damaged Area

This is the most important part of the repair process and taking short cuts here is the main reason why bumper repairs fail so grab some sandpaper and channel your inner Mr. Clean.

Clean with Plastic Solvent

Scrub the repair area with plastic surface cleaner or prep solvent using a Scotch-Brite pad. A simple wipe-down with a rag isn’t going to cut it. Be sure to get the back side and edges of the crack. This will remove contaminants from the surface so you don’t sand them in during the next step.

If your bumper cover is really gritty, wash the repair area inside and out with soap and water before scrubbing it with plastic surface cleaner. Lacquer thinner works well too for tough grime.

Sand with 80-grit Sandpaper

Sand down the repair area starting with 80-grit sand paper on a DA and remove the paint from at least 2 inches around, feathering the edges so they transition seamlessly from dull to shiny paint. Then smooth out any rough or jagged edges along cracks and tears by hand with sandpaper.

Grind a V-groove Along the Crack

Without punching all the way through, gently carve along the edges of the tear to create a bowl or river. You don’t want the groove to be a sharp beveled edge but rather a shallow incline that gradually flows down at an angle for about 1/2″. This provides additional surface area for the adhesive to grab on to for a stronger repair while making it easier to align the two edges. You should also do the same on the backside of the bumper cover.

For plastic types that powder when sanded, cut the grooves with a 24-grit disc on a sander. For plastics that smear when sanded, make the grooves using a cordless drill and a rotary file. This gives you greater control and slower speeds to reduce the chance of heat-generated smears.

Optional Extra Step: Drill Holes Along Both Sides of the Crack

For larger cracks, drill a series of 1/8″ holes at regular intervals spaced about an inch apart going down the length of the tear on both sides. These holes act like rebar in cement, adding yet another place for the filler to grab onto.

Clean It Again

I know that at this point you’re anxious to start your repair but it’s important to thoroughly wipe down both sides of the bumper cover with plastic solvent using a Scotch-Brite pad one more time to clean up any remaining sanding dust and remove mold release that may have been freed in the process. You now have about 30 minutes to start applying your products because not too long after that, mold release will start to emerge again. The faster you start the repair after the final cleaning the more durable it will be.

4. Repair the Back of the Bumper Cover

Always start your repair from the back if you have tears and cuts that go all the way through the bumper cover. To keep the two pieces of the tear from shifting around, use tape across the front to hold the edges of the tear together and keep them aligned while you work.

Apply Adhesion Promoter

Primers don’t usually bond to plastics very well so the adhesion promoter acts like a middle man to join the two opposing parties. Thoroughly spray or wipe your adhesion promoter over the entire repair area making sure to cover the inner edges of the crack. Wait about 10 minutes for it to completely flash (check your product for specifics).

Apply a Mesh Patch

Cover the tear on the backside with a self-stick re-enforcing mesh patch that spans out at least 1 inch from the tear. Press down firmly so every bit of the patch is making contact with the bumper’s surface.

Mix & Apply the Flexible Epoxy Adhesive

Squeeze out an equal amount of the 2-part flexible epoxy adhesive onto a piece of clean cardboard and mix it thoroughly with a body filler applicator until the color is uniform. If your cartridge includes a long mix nozzle for application be sure to equalize the two sides of the cartridge first by purging a little until both sides are coming out equally.

Apply the adhesive mix to the tear on the backside with either the tip of the mix nozzle or a body filler applicator so that the material oozes through the mesh to fill the groove and rebar holes (if you drilled any). Don’t worry about being neat – no one will see your globby mess back there. Build up the surface while the mixture is still wet in 3-4 wipes and then let it dry for about 20 minutes.

5. Repair the Front of the Bumper Cover

Once the material on the back has hardened, move to the front of the bumper and remove any tape you used to hold the tear together while working on the backside. Sand down any excessively large blobs that may have seeped through the crack with 80-grit sandpaper.

Apply Adhesion Promoter

Just like you did on the backside, liberally spray or wipe on your adhesion promoter over the repair area on the front of the bumper cover. Let it dry for about 10 minutes or amount of time indicated on the product.

Mix & Apply the Flexible Epoxy Adhesive

Using the same technique as before, apply the adhesive mix and push it down into the groove and holes (if you drilled some) with a body filler applicator so it meets up with the hard epoxy from the backside. Spread the adhesive mix the same way you normally apply Bondo. For big repairs, it’s better to use small amounts and build up in several layers, allowing it to dry between each layer than filling a giant crack with a huge blob that won’t hold its shape as it dries. This technique will make it easier to contour later.

6. Sand & Contour the Surface

The adhesive mix should to ready to sand in 20-30 minutes but check the recommendations on your product. Start with 80-grit sandpaper on a sanding block to knock down the high spots and rough edges then switch to a DA sander. When you’re happy with the shape, smooth out the entire area with 150 or 180-grit sandpaper so that when you run your fingertips from the center of the repair to the edge you don’t notice any bumps, scratches, dips or a lip when you reach the good paint. I like starting with a DA and then finishing it up by hand blocking until absolutely smooth.

The adhesive filler should transition seamlessly into the rest of the bumper. If you accidentally sanded away too much and have a low spot, clean the area again with adhesion promoter and apply another layer of adhesive mix. Use plastic finishing putty or filler to cover up any pin holes on the surface for a smooth finish that’s now ready for primer.

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