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C3 Corvette Bumper Repair - Rubber Revamp
Simple Fixes For Factory Urethane Bumper Caps
Mar 11, 2010
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C3 Corvette Bumper Repair - Rubber Revamp
Our '76 shark is fitted from the factory with urethane bumper covers. While 30 years of service is commendable, the battle scars were evident. The worst of it was the right corner, which showed evidence of cracking from a minor impact.
Our plan was to repair the cracks, and repaint the upper and leading surfaces of the original covers. Repairing the cracks begins with "V-ing" out the damaged area along the length of the crack. This gives the repair compound a broader surface area to grab, which will result in a stronger repair. We used a carbide burr, though a grinder or even sanding can accomplish the same effect.
Note that our V-groove runs the full length of the crack, and the cut is well into the thickness of the urethane cover, but not all the way through.
To protect the paint beyond the bumper cap, we masked the nose with a heavy tape line to the seam where the cap meets the body. This will save blending the new paint we plan to spray on the cap into the adjacent fiberglass. Even taped, sanding will require extreme care near the edge to avoid cutting through the paint we want to save.
Since the paint on our cover was basically shot, we stripped the worst of it for a repaint. Careful sanding with an autobody D/A sander and some detailed hand sanding quickly had the old finish removed.
A cold-welding compound, basically a highly specialized filler/glue, is a convenient and effective method of repair in this application. SEM No. 68442 Bumper Repair compound is made for the job. The catalyzed product comes in a dispenser that meters equal portions of the two components.
The repair material is applied much like body filler, but the working time is relatively short. Apply the filler a little proud (sticking up higher than the original or surrounding surface) of the original surface, but avoid overworking-aim to lay it down it in a few quick swipes.
Once cured, the SEM Bumper Repair compound can be sanded to shape. We found the material conformed well, and there was no evidence of it peeling at the sanded edges, indicating an exceptional bond.
To fill minor imperfections, we turned to a two-component glazing putty from Evercoat. These resin-based fillers are smooth sanding and flexible enough in thin layers to be useful in this type of repair.
A quick swipe of the glazing filler was enough to bury the remaining imperfections and the low spots left after sanding the bumper-repair compound.
Sanding with No. 150-grit paper gave us a smooth and well-feathered repair. At this stage, the offensive cracks in our bumper cover are gone.
Next, we prepared for refinishing. After stripping the paint from the opposite side, the nose of our Vette was carefully masked. Note that some additional filler was applied to the right side of the cover. This was to reduce some of the most objectionable waviness, a characteristic of these urethane covers.
A primer coat is needed to achieve a surface smooth enough for final paint. We used Nason 2K urethane, which is a high-build and fast-curing primer surfacer.
The primer was done in two stages: shooting the cover with two good wet coats, and then, once cured, sanding with No. 220 dry. This was followed by a second application of primer, consisting again of two coats.
To fill minute imperfections, such as sanding scratches or pinholes that inevitably appear once the primer is sprayed, we used regular air-dry acrylic spot putty.
A guidecoat is a contrasting color sprayed in a light fog over the primer coat. We used some surplus orange basecoat we had, which makes an easy-to-see guidecoat over gray primer.
Our second primer application was sanded with No. 320 dry, using a flexible hand block and light pressure. The urethane bumper cover is flexible, so sanding requires a light touch and sharp, fresh abrasives. Note how the contrasting guidecoat shows when the surface has been sanded to a smooth, even finish. Once fully sanded, we sanded again with No. 600 in preparation for paint.
We had the factory white hue mixed to code at a local paint supply, selecting single-stage DuPont Centari acrylic enamel. A universal flex additive can be added to the paint to make it more pliable on the flexible urethane bumper cover.
We shot the cover with a light tack coat first, followed by two medium wet coats. The white paint covered quickly and flowed out nicely, providing an OEM look.
A close look fails to reveal any hint of the damaged right corner's cracks after the repairs were made.
Overall, fixing the bumper cap gave our old C3 a much cleaner look. Though the paint on this car is far from perfect, the cracked and peppered bumper was by far the worst looking area. A few minor repairs can make a big difference.
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