Dealing with cracks in fiberglass is no fun, but unfortunately it is a necessary part of owning a fiberglass-bodied car, especially a classic like a C2 Corvette. The C2 was built from 1963 through 1967 and is considered by many to be the ultimate in Corvettes. The problem is that the very nature of fiberglass is that it never stops shrinking. Combine this with years of hard road use, and there are bound to be stress fractures.
While working on a 1966 Corvette coupe, we found a few spiderweb cracks while we were stripping away the paint under the hood. We could spend a lot of time and money replacing the panels or we could repair them. We had been looking for a reason to try out the JB Weld Plastic Bonder we found at our local parts store, so the choice was made.
The Corvette bodies were not created in a single mold, instead they were built from multiple pieces, bonded together, using the first type of structural body adhesive, it was cutting edge technology at the time. Modern cars are almost completely glued together for the most part, using modern structural body adhesive epoxies. JB Weld Plastic Bonder is essentially the same stuff, just in an easy to use syringe dispenser. Structural body adhesives are so strong that the material they are bonded to often will fail before the epoxy does. Plastic Bonder is a 2-part epoxy that bonds to just about anything, but it is specifically designed for plastic and fiberglass. You can even use it to bond metal to plastic or fiberglass, which is really handy when you need to add some support.
To get the process going, we need a few tools. While you could certainly just glue whatever parts together and make it work, we want a repair that not only functions well, but looks good also.
1/8” drill bit
Spreader (metal or plastic)
Die-grinder with a carbide burr (optional)
Time- 1 hour work time, 24 hour wait time, plus refinishing.
Difficulty- On a scale from 1-10, 10 being the hardest, 3
The carbide burr and die-grinder are optional, but for the kind of repair we are doing, they will help us get a better result. If you don’t have one, you can always get similar results with a triangle file or even a piece of coarse sand paper (36-80-grit). More on that later.
The process begins by locating the cracks and then finding where they start and stop.
Each crack gets drilled at the end points. This is to ensure that the crack stops growing. The size of the drill bit will be determined by the size of the crack. Wide splits will need a larger bit.
Next, the cracks were grooved with the die-grinder. The epoxy needs a place to bond to the material, and the more epoxy you can fit into the crack, the better. We are not cutting very deep, about half the thickness of the base material.
The next step would normally be to sand the area down for a good adhesion surface, but the Corvette panels have already been sanded, that is how we found the cracks in the first place. At this point, the area was wiped down with some 3M paint prep and it was ready for the epoxy.
Using a mixing board, a bit of epoxy was pushed out and then mixed up with a spreader. We are using a metal spreader because they clean easily and are more sturdy than plastic. That helps when you are pushing the material into a crack. Only mix what you can use in a few minutes, the epoxy has a work time of about 5 minutes.
The epoxy was spread over the cracks and pushed into the crevices. You want to leave extra on top of the repaired area to be sanded off later; otherwise you will have to do more finish work.
The epoxy takes 15-20 minutes to harden, but we let our repairs sit overnight to fully cure before sanding. We did have a batch that never fully hardened, instead it stayed rubbery. This was the first mix out of the tube; we suspect it was improperly mixed.
Next, the entire area was sanded with 12-grit sandpaper until it was level with the original surface. The epoxy sands very easily, resulting in a smooth finish that did not require any additional finish work. At this point, the repairs are done.
To finish off the underhood area of the Corvette, we needed to paint it. The repairs will show through regular paint, so the area was sprayed with a primer sealer. This covers all repairs and provides a uniform surface for the paint.
Then we sprayed some single-stage semi-gloss paint.
The results of the JB Weld Plastic Bonder repair were excellent. Not only was the process quick and easy, but the panels look like they did from factory. The bond is strong; we would use this stuff on just about any plastic or fiberglass component that needs to be repaired. In other words, you can get professional results when you take the time to do it right.