How to Repair a Polyurethane Bumper

by Chris Stevenson
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Most late model automobiles have plastic bumpers that are able to absorb mild impacts. When hit hard enough, however, these bumpers can split and crack, resulting in unsightly damage. Common to many models, polyurethane bumpers have stamps on them that denote their chemical makeup. A "PUR" (polyurethane rigid) stamp on your bumper means it should be treated with a special repair process that allows proper adhesion and flexibility. Most auto parts stores sell kits that enable car owners to complete this work themselves.

Step 1

Remove the bumper from the vehicle, if you haven't already done so. Use the correct sockets and wrench. Some bumpers require loosening several screws in the back bumper plate, then sliding the bumper off as a separate unit. Consult your owner's manual for the proper location of the screws, fasteners and brackets. Once the bumper has been removed, clean it inside and out with soap, water and a sponge. Dry it with towels.

Step 2

Set the bumper on a bench, backside facing up. Scuff up the crack area with 80-grit sandpaper, overlapping the crack area by at least four inches. Wipe it down with a rag soaked in plastic surface cleaner from your kit. Dry buff it with a clean towel. Use another rag that is wet with prep solvent from your kit to wipe down the crack area. Wipe in only one direction, not back and forth. Complete the exact same steps with the front side of the bumper.

Step 3

Affix a rotary drill bit to a die grinder. Carefully cut a U-shaped bevel in the crack from one end to the other on the backside of the bumper. Flip the bumper over and do the same to the front side. The bevels will allow more surface area for the plastic filler. Clean both areas again, starting with the plastic cleaner and drying it. Then use the prep solvent on a rag to wipe in one direction across the crack area.

Step 4

Make sure the bumper sits front side up. Apply plastic adhesive tape from your kit across the length of the crack and press it down firmly in place. Use as much tape as you need. Flip the bumper over. Consult the directions for the plastic filler that came with your kit. Mix in equal parts of the hardener and adhesive on a scrap of cardboard or paper and swirl the formula with a stick until it mixes thoroughly.

Step 5

Scoop the plastic filler mixture with a putty knife and trowel it into the grooved crack (backside of bumper). Build up the height of the plastic filler by spreading it thick. Overlap the crack at least four inches. Let the plastic filler dry and cure for about 30 minutes, or according to your directions.

Step 6

Flip the bumper so it rests face up. Remove all the tape from the crack surface. Mix another batch of plastic filler on a piece of cardboard or paper. Use the putty knife to trowel plastic filler into the crack. Build up the height of the plastic filler so it sits slightly higher than the bumper's surface. Overlap the crack by at least four inches. Let it air dry and cure according to directions.

Step 7

Use the heavy 80-grit sandpaper, wrapped around a sanding block, to sand over the crack area. Use smooth, even back-and-forth strokes, but don't sand all the way to the surface level. If the crack rests on a curve or seam area, place the sandpaper in your palm or wrap it over a stiff sponge, so it follows the contour of the bumper surface.

Step 8

Use 120-grit sandpaper to take down the surface further, sanding as you did with the 80-grit. For the final finish, use the 400-grit sandpaper to sand the surface down level with the bumper's surface. When satisfied with the final sanding, wipe it down with plastic cleaner. You can now prime and paint your bumper accordingly.

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