How to Repair "Spiderweb Cracks" on Polyurethane Bumpers

Although urethane bumpers are quite rare on late-model vehicles, the need may arise for one to be repaired from time to time. This bumper belonged to a 1992 Chevrolet Camaro, and, as you can see, it has developed what we call "spiderweb cracks". These cracks are the result of dirt and grime settling inside cracked paint and eating away at the plastic over time. Let's take a look at how to repair these cracks using products in Polyvance's plastic repair line.

Polyurethane is a thermoset plastic, meaning that it is NOT meltable. The solid is formed by reacting two liquid components which crosslink in the mold. You will NOT be able to use the nitrogen welder on this type of plastic. You can repair it with the airless plastic welder. This will not provide a fusion weld. The urethane welding rod is used more like a hot melt glue.

You can use our Mini-Weld Model 7 or the integrated airless welder that is part of our nitrogen plastic welding systems to perform the repair. These two options are recommended because you need to be able to control the temperature of the welder.

This article can help you determine if your plastic part is thermoset polyurethane or a thermoplastic.

Always wear proper safety gear while working!

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Products Used:

Note: This is a list of the Polyvance products that can be used to make this repair. This list does not include sanders, grinders, or other common tools you will need. If you have any questions, please don't hesitate to call technical support at 800-633-3047.

• 6122 Heavy Duty Round Cutting Burr
• 2000 Flex Filler (several options available)
• Waterborne Primer Surfacer (several options available)
• Polyurethane Welding Rod and an Airless Plastic Welder (choose a method below):
• Polyurethane Welding Rod with the airless plastic welding side of a nitrogen plastic welder. Our current nitrogen plastic welders include the 6027HT Standard Welding Tip.
• 5700HT Mini-Weld Model 7 (230V kit available here). The 5700HT kit includes a 15 foot starter roll of polyurethane welding rod and the 6027HT Standard Welding Tip.
• Note: Make sure you are using either the 6027HT Standard Welding Tip or the 6029 Bigfoot Welding Tip when using the round polyurethane welding rods.

Video Notes:

• This type of repair can be done with the Mini-Weld Model 7 or the airless side of a nitrogen plastic welder.
• English Closed Captions have been provided for this video. Click on the CC button to turn them on or off.
• The length of this video is in no way representative of the actual time required to perform a complete repair and therefore should not be used for estimating purposes.

English Transcript:

Although urethane bumpers are quite rare on late-model vehicles, the need may arise for one to be repaired from time to time. This bumper belonged to a 1992 Chevrolet Camaro, and, as you can see, it has developed what we call "spiderweb cracks". These cracks are the result of dirt and grime settling inside cracked paint and eating away at the plastic over time. Let's take a look at how to repair these cracks using products in Polyvance's plastic repair line.

STEP ONE: Using 80 grit sandpaper and a six-inch sander, sand away the paint from the area you're going to repair. Once the area has been sanded, push up on the underside of the bumper in order to see the depth of the cracks. If the cracks are only surface-level scratches, you WILL NOT have to do any grinding into the plastic, and you can now skip ahead to STEP SIX. These cracks, however, will need to be sanded due to their varying depths.

STEP TWO: Using 36 grit sandpaper, grind away at the plastic until the shallow cracks have been removed. As you can see, all that remain are the more severe cracks, which will need to be welded.

STEP THREE: Using a 6122 round cutting burr, v-groove into the deep cuts until the cracks are gone.

STEP FOUR: Now, using one of Polyvance's airless plastic welders, melt R01-01 Polyurethane welding rod into the v-groove until it is filled out to the surface. Since these cracks are not all the way through the plastic, we will only need to do a front-side repair. If both sides had been cracked, we would have repeated the process on the other side as well, filling both sides with the melted welding rod.

STEP FIVE: Allow the melted plastic to cool completely, then use 36 grit sandpaper to shave the weld even with the existing plastic, then sand again with 80 grit sandpaper.

STEP SIX: Mix together 2000-T Flex Filler, making a uniform grey color. Smear the Flex Filler over the welded area, making sure to fill any pinholes that may cause air bubbles.

STEP SEVEN: Allow the Flex Filler to cure, then, using 80 grit sandpaper on a DA sander, grind the filler until it is even with the existing plastic.

STEP EIGHT: To further insure that the filler is even, sand again using 80 grit paper and a sanding block.

STEP NINE: Once you have block sanded, smooth the repair area again, using 180 grit sandpaper to remove any sand scratches made by the block sander.

STEP TEN: Next, prime the repair using one of Polyvance's waterborne primers. Allow to dry, then block sand again using 320 grit sandpaper to blend. Repeat this process until you are pleased with the texture and appearance of your repair.

As you can see, even tricky polyurethane repairs like this one can be simplified using Polyvance's comprehensive line of plastic repair products.

Look to Polyvance for all of your plastic repair product and training needs. We offer I-CAR Industry Training Alliance courses as well as other hands-on classes in addition to the video resources you can find on our website and on our YouTube channel.