Fix a cracked bumper
So you hit the parking ramp barrier and now you have a cracked bumper? The body shop wants a fortune and you want to fix a cracked bumper yourself. You’ve searched the Internet and watched all the youtube videos that show you how to fix a cracked bumper yourself. DON’T DO THAT! If you follow the instructions in those videos you’ll be wasting your time and energy. All the posts and videos show the same thing; using epoxy and Bondo to fix the crack. Those products won’t last and they’re incompatible with the plastic used to make the bumper cover. Epoxy and Bondo are rigid materials. But your bumper cover is made from a flexible plastic. If you use those products, they’ll break off the instant your bumper flexes. The crack will open up again and you’ll have to redo the repair from scratch.
What you need to repair a cracked bumper
If you want to repair your cracked bumper the right way you’ll have to spend some money on a crack stitching tool and a special plastic welding kit. A plastic welding kit contains a hot iron that will melt the bumper plastic but not destroy it. You’ll also have to remove the bumper from the vehicle, so you should have access to a shop manual to locate all the bumper cover fasteners. Try alldata or Mitchell (see the links below). Next, take a moment to understand bumper construction.
Understanding bumper construction
The bumpers on modern vehicles are made with three pieces; the bumper cover (which is the part that cracked), the energy absorber (which probably also got damaged during the impact) and the impact bar (the actual bumper). Here are illustrations for a front and rear bumper from a 2015 Toyota Camry. You can see that the bumper has three components: bumper cover, energy absorber, and the impact bar.
The bumper cover material is made from flexible plastic. But car makers use a variety of plastics to form their bumper covers and the filler plastic you use to repair the crack must match the base material. If you use any other type of filler to fix a cracked bumper cover, it won’t last. It will simply crack off. Here’s a list of the most popular materials:
TPU TPUR = Thermoplastic Polyurethane
It’s used in bumper covers, soft filler panels, gravel deflectors, rocker panel covers.
TPO, EPM or TEO = Polypropylene + Ethylene Propylene rubber
It’s used in bumper covers, valence panels, fascia panels, air dams, dashboards, grilles.
It’s used in bumper covers, deflector panels, interior moldings, radiator shrouds, inner fenders
PC + PBT= Polyphenylene Ether + Polyamide Bumper covers
If you use any other type of filler to fix a cracked bumper cover, it won’t last. It will simply crack off.
Do some Internet research or use a shop manual to determine the type of plastic used in your bumper cover.
Steps to fix a cracked bumper
The advice on the Internet shows how to patch the bumper crack by first removing the bumper and applying 2-part epoxy covered with fiberglass cloth or drywall mesh. Trust me, that method won’t work. Here’s why. Drywall mesh isn’t strong. Don’t believe me? Just try pulling it apart with your fingers. The mesh comes apart in your fingers. If you’re going to use any kind of backing material with the crack repair, at least choose a stronger fabric like fiberglass cloth or stainless steel screen. They offer lateral support that drywall mesh doesn’t.
Stitching the crack from the backside is the first step in the crack repair. There are two ways to do this and both require you to remove the bumper from the vehicle.
Stitch a cracked bumper with hot staples
The hot staple method is the fastest and most long-lasting. Start by clamping cleaning the cracked edges with a wax remover. Then close the gap between the two cracked sections and clamp them together using small wood blocks and a vice grip type locking pliers or wood clamp.
Next, load a staple into the heat gun and press the trigger. Wait
for the staple to get red-hot and plunge it into the middle of the crack to a depth of no more than half the thickness of the plastic. Allow the staple and plastic to cool before releasing the staple from the gun. Repeat the operation by alternating the staple rotation direction. Once you’ve stitched the crack, cut the protruding staple legs with a side cutter.
Find the Astro 7600 Hot Staple Gun Kit for Plastic Repair Kit on Amazon for about $90. It comes with the heating gun and an assortment of stitching staples. See this video below for usage.
Fill the cracked bumper from behind with plastic welding rod
Just like with hot staple stitching, you must remove the bumper cover from the vehicle to stitch the crack with plastic welding rod. Instead, purchase a plastic welding kit like the Polyvance 5700HT Mini Weld Model 7 Airless Plastic Welder. The kit comes with all the types of bumper cover plastics commonly used by car makers
6027 HT Standard Welding Tip
6031 Tear Drop Airless Welding Tip
200 Watt Ceramic Core Heating Element
2045W Reinforcing Mesh
6124WB Stainless Wire Brush
Starter Pack (15 ft) of round Polyurethane Welding Rod
Starter Pack (15 ft) of round Polypropylene Welding Rod
Starter Pack (15 ft) of round ABS Welding Rod
Starter Pack (15 ft) of round LDPE Welding Rod
Starter Pack (15 ft) of round TPO Welding Rod
Starter Pack (15 ft) of round Nylon Welding Rod
Starter Pack (4 ft) of round Polycarbonate Welding Rod
Starter Pack (15 ft) of flat FiberFlex Universal Welding Rod
Follow the instructions in this Polyvance video showing how to stitch the backside of the bumper with stainless steel screen and groove and fill the front side using FiberFlex filler material
Prime with a flexible primer
Once you complete the plastic repair and the surface is sanded smooth, apply an
automotive-grade flexible primer like SEM 39133 flexible primer (semproducts.com).
Paint the cracked bumper with automotive paint with flex additive
Locate a professional auto body paint supplier that has the capability to load your paint into an aerosol can along with a flex agent. That will allow the paint to flex and not crack off like ordinary touch up paints.
©, 2019 Rick Muscoplat