by Daniel Burrill with Jeffrey Zurschmeide
(Taken from How to Fabricate Automotive Fiberglass & Carbon Fiber Parts by Daniel Burrill with Jeffrey Zurschmeide)
Fiberglass automotive bodies are lightweight and resilient but not indestructible. When a fiberglass body makes contact with a solid, heavy object, it doesn’t bend, divot, or crease like steel automotive bodies; rather the fiberglass body cracks or breaks. The materials and techniques for repairing fiberglass are far different than ones used for repairing sheet metal.
This project fixes a crack in the fiberglass body of a 1977 Chevrolet Corvette. The crack begins on the top of the right rear fender and has propagated under the paint to the apex of the right rear wheel arch. The crack is in the actual fiberglass panel, so it must be repaired and cannot be smoothed over with filler. In fact, someone did smooth it over with filler in the past, and the crack has continued to worsen under the paint!
About $50 in materials, assuming you already own the tools.
• Dual-action sander and sanding discs from 80 to 200 grit
• 4½-inch body grinder
• Long-block hand sander
• Halogen shop light
• Plastic spatula
• Fiberglass cloth
• Resin and catalyst
• Duraglas body filler
• Plastic mixing cups, wooden stirrers
Time to Work:
About 8 hours, spread over several days
Follow These Steps:
1. The crack in this Corvette’s fiberglass is on top of the right rear fender. Because of the location and severity of this crack, we need to get access to the underside of the fender. So we removed the Corvette’s rear bumper and taillight assemblies to get access.
Where the paint has chipped away, you can see a crack in the fiberglass. This needs a proper repair or it will continue to come back, no matter how many coats of paint you put over it.
When you’re working with a crack in fiberglass, you usually have to sand enough around the flaw to find out how far the crack extends. This requires filling and smoothing when you’re done.
3. When we got the back end of the car removed, we were able to look at the backside of the crack, and of course, we found a big patch of old body filler stuck to the underside of the fender. This was not a very effective repair. It’s the equivalent of treating a broken bone with a heavy coat of makeup.
You can see the Bondo slopped onto the backside of the crack; this does nothing to fix the problem, however.
5. Working from the back or underside of the fender, lay down a layer of resin and then fiberglass cloth with more resin to support both sides of the crack. Because this area is hidden, you can apply as many layers as necessary (two or three should be plenty) to support the sides of the crack. Fiberglass reinforcing mesh sold for wallboard repairs is a great choice for a supporting material.
Here’s the repair on the underside of the fender. This layup holds the two sides of the crack together and supports the surrounding area.
6. On the top side of the repair, apply resin and a single layer of fiberglass cloth, and then leave the project to set up overnight. To make sure everything stays warm, you can use a basic halogen work light from the discount tool store and place it inside the fender on the frame rail. This keeps your new fiberglass toasty warm if it’s cold in your garage. Just be sure that your glass doesn’t get too warm so you don’t create a fire danger. Check on it frequently.
7. After the basic fiberglassing is done and cured, grind the topside of the repair level with the surrounding bodywork, and then mix up and apply some Duraglas or other fiberglass-based body filler. When this material has cured, you can use the dual-action to sand it smooth, still being careful to respect the cut-lines in the bodywork.
The fiberglass cloth splint has been placed over the outside of the crack. We’ve already sanded it down level with the surrounding bodywork.
After another round of sanding, you can apply some Duraglas body filler to the area. It is the new pink filler in the center.
When everything is done and smooth, hit the area with one last coat of primer so it’s ready to paint.