In 2007 Jeep and its supplier, ABC Group, were recognized by the Society of Plastic Engineers for incorporating blow molded textured plastic bumpers on the JK-series Jeep Wrangler. These blow molded bumpers replaced Jeep’s long running rolled formed steel bumpers. Replacing the steel bumper with a blow molded plastic bumper offered a 12 percent reduction on piece cost and assembly cost, as well as a 9 percent weight reduction, compared to the previous steel bumper.
Blow molded parts are nothing new to the automotive industry. However, using this blow mold process to make a plastic bumper is new. And it might seem somewhat challenging to repair and remove a dent from a bumper like this, since you can’t push a dent from the backside. Luckily these bumpers are hollow, and depending on how severe the damage is, it’s actually not that difficult if you have the right tools and equipment.
Before repairing any part, clean the bumper before starting the repair process. Washing molded-in-color textured plastic bumpers is more important than normal because they are made of raw TPO plastic and people often use silicone-based products to restore the shine. It is important to wash with soap and water two or three times, allow to dry, and clean again with a solvent based plastic cleaner to ensure the bumper is free of any contaminates.
The key to removing a dent in a blow molded bumper is to cut an access hole on the backside. The removed panel will be welded back in to seal the bumper after the dent is pushed out. Depending on where the dent is on the bumper, it may be difficult to access from the backside where the bumper mounts, for example. Before you decide to attempt the repair, make sure you have clear access to the back of the dent in the area you plan to cut the access hole.
If you want to attempt the repair, cut the access hole on the backside large enough to give you complete access to the back of the dent. Mark where you will cut the hole on the back of the bumper with a marker, then carefully cut it out with either a jigsaw or a sharp utility knife. Reserve the removed access panel for reattachment later on. After you cut the hole, reach in with your pushing tool to make sure you can access all angles before heating the part. Cut the hole bigger if needed.
Start heating the dent with a heat gun. Be patient and let the heat sink into the plastic. Heat from the frontside only until the plastic on the backside is too hot to touch. Be sure to heat at least four inches around the outer ring of the dent to unsure all the tension is released when you push the dent out. As you heat the bumper, push up on the dent frequently to make sure you don’t over or under heat the plastic. These bumpers are made from fairly thick TPO plastic, so the reshaping and manipulating of the plastic is a little easier than thinner bumpers.
When working dents out of bumpers, the general rule is to keep pushing the low spots up from the backside until the profile feels about right. Push any high spots down as needed to get the profile perfected. Let the plastic cool completely, then sand the dented area with 80 grit in a DA sander. Sanding the area will immediately reveal the remaining low spots. Try to sand it to the correct profile, but it’s most likely you’ll have to heat the bumper again and push out the low spots and push down on the high spots again. Often you’ll have to repeat this pushing and sanding process three or four times until you get the correct profile.
Most often some filler will need to be applied to perfect the bumper’s profile. Follow your filler manufacturer’s instructions as to the need for adhesion promoter and surface preparation. Most often, an adhesion promoter will have to be applied to the TPO plastic prior to the application of any coating, either filler or primer. Allow the filler to cure completely, then sand and feather with 80 and 180 grit sandpaper.
Before proceeding with the refinishing and retexturing process, weld the access panel back into the hole you made on the backside of the bumper. First, use a rotary tool to grind a bevel all the way around the edge of the hole and around the edge of the panel. This will create a v-groove for the welding rod to go into. Once the panel is prepped, apply some aluminum tape inside the bumper at the corners of the hole with the sticky side facing up; these will prevent the access panel from falling through the hole. Before finishing the v-groove around the panel’s perimeter, tack weld the panel’s corners to keep it from popping out if the rotary tool snags an edge. You may also use a hot airless welder tip to melt the v-groove in.
Use polypropylene or TPO plastic welding rod with a nitrogen plastic welder to weld the access panel back into the bumper around the perimeter of the panel. If the groove was deep enough, the weld can be finished smooth with an airless plastic welder, which will minimize any finish work that needs to be done on the backside.
The final step of the process is to refinish the textured bumper. To achieve a uniform appearance overall, It is easier to retexture the entire bumper instead of trying to blend the texture. The first step in retexturing is to sand the entire bumper with 180 grit paper in a DA to smooth the texture. It’s not necessary to completely remove the texture; you just want to take the tops off the texture.
Once the plastic is sanded overall and blown dust free, apply an adhesion promoter overall and allow to flash per manufacturer instructions. Then apply one coat of high-build primer surfacer and allow to dry. Sand the primer with 320 grit paper and try to achieve a uniform appearance between the dented area and the undamaged bumper. Fill any minor imperfections with spot putty and apply additional primer coats if needed to achieve a uniform smoothness over the entire bumper.
Several manufacturers make texture sprays for refinishing textured bumpers and trim. Use the manufacturer’s instructions to adjust grain size to match the original as closely as possible. Usually texture coatings are applied in thin “splatter” coats by increasing the distance to the work and the speed of the pass to prevent the paint from flowing out. Several light coats are applied to achieve the desire appearance. Finally, once the grain looks acceptable, color coat if necessary to match the original color.
