How to Repair a Crack in a Corvette's Fiberglass Body

Corvette Fiberglass Crack
Here's the crack in the fiberglass of the Project 1977 Corvette. We'll show you how we fixed it!. Photo by Jeff Zurschmeide

One of the projects that every restorer of an old Corvette eventually has to tackle is a crack in the fiberglass. Corvette bodies are made entirely of fairly thin fiberglass, and the sensuous curves of our cars are what gives the bodywork some cross-section to maintain its rigidity. It feels like the bodywork is more substantial than it really is. Yet when you're driving, your Corvette flexes all the time. Eventually, it can crack. Cracking is virtually assured if your body mounts are compromised or the car has been hit. Wheel arches are always in danger of cracking due to stones that get flung up by your tires and hit the fiberglass like bullets.

This project fixes a crack in the fiberglass bodywork of a  1977 Chevrolet Corvette. The crack was on the top of the right rear fender in the actual fiberglass panel, so it had to be repaired and couldn't 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!

To do a job like this, you'll need a dual-action sander and sanding discs in a variety of grits from 80 to 200. You might also need a 4.5-inch body grinder, depending on how much Bondo has been used in the past. Get a longboard hand sander and a bunch of sandpaper from 80 to 200 grit or so. A halogen shop light is handy for both light and heat. And you'll need a plastic spreading spatula for Bondo, as well as scissors, brushes, a fiberglass roller, and some disposable cups for mixing fiberglass resin and other materials. You'll want a supply of fiberglass cloth, resin and catalyst, Bondo, and high-build primer, too.

This project takes several days to complete but can be done in perhaps eight hours of actual work. You need to leave time for the resins and Bondo to harden between steps. You can choose to do this work yourself, but many readers may review the procedure and decide to leave this kind of work to the Corvette body and paint professionals. In that case, you'll be able to talk about the work knowing what's really involved in the process.

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Find Out How Bad The Crack Really Is

Corvette Crack Repair
We sanded away the paint and bondo to find out how big the crack really is. Be careful of those nice sharp Corvette creases!. Photo by Jeff Zurschmeide

Because of the crack's location and severity, you will need to get access to the underside of the fender. In this project, we removed the Corvette's rear bumper and taillight assemblies to get access. This ended up being a good thing because we found leaky old fuel lines back there!

While we were removing the rear end of the car, we also used our D-A sander to scrub off the paint around our crack and found that the crack had been covered over with Bondo and paint before, and it had flexed enough to create another crackdown at the wheel arch.

Note that when you use a D-A or any sander or grinder on fiberglass, you absolutely have to be careful to respect the creases and cut-lines in the car's bodywork. If you grind down a convex crease, you'll need to rebuild it with filler and carefully reshape it -- and it's far easier to just be careful around those features!

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Look At The Backside

Corvette Crack Repair
Here's the old bondo job that didn't really fix the crack. We'll grind it away and add some fiberglass cloth to make this a better repair. Photo by Jeff Zurschmeide

Once the back end bumper cover was off, we were able to look at the back side of the crack and found a big patch of Bondo stuck to the underside of the fender. This is about the equivalent of treating a broken bone with makeup. Bondo fills the crack but has only minimal strength under tension, so it cannot really "bridge the gap" of a crack.

Much of the Bondo was ground away, and then a fiberglass cloth patch was applied to the backside of the crack to give it as much real support as possible.

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Repair the Backside

Corvette Crack Repair
Here's what the layup looks like from the underside of the fender. This will give the repair some strength so the crack doesn't open up again. Photo by Jeff Zurschmeide

To fix the crack, we first ground away material around the crack from the topside with the D-A sander, and we used a body grinder to get rid of the Bondo from underneath, being careful not to do more damage to the fiberglass of the body. 

They then applied fiberglass cloth with resin to support both sides of the crack. On the top, they applied one layer of fiberglass cloth. Those were left overnight to set. Use a basic halogen work light from the discount tool store and place it inside the fender on the frame rail to help it stay warm and set. This kept the new fiberglass toasty warm while the resin hardened.

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Fix the Top Side of the Crack

Corvette Crack Repair
Here's the glass patch we put on the topside of the crack, all sanded down smooth with a small amount of Bondo body filler. Photo by Jeff Zurschmeide

After the basic glassing was done and cured, the topside of the repair was ground down. Then high-tech Duraglass body filler was applied. It was sanded smooth. 

Once the basic shaping was done, the repair team made similar repairs to the crack propagation on the side of the fender and down at the wheel arch. The same techniques apply -- a layer of glass cloth spanning the crack, then sand that down and use a thin layer of filler to smooth everything out.

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Sand the Body Filler

Corvette Crack Repair
A thin coat of body filler helps smooth out our repair. Now we'll use that long board and make sure the whole thing is factory-smooth and ready to look great!. Photo by Jeff Zurschmeide

Body filler works like fiberglass resin; you add a catalyst and the plastic resin sets up hard over the course of 15 minutes or so. Only mix up what you can use in that time. You want to get a very thin layer over your repair. Be sure to work it into the low spots with your plastic spreading spatula.

When you have the filler spread out and it has hardened a little bit, you can use your heavier grit sandpaper to grind the material down to body height. The goal is to get the filler perfectly level with the surrounding fiberglass.

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Prime and Paint

Corvette Crack Repair
Here's the finished repair, primed over and ready for paint. Photo by Jeff Zurschmeide

Once the repair site was smooth, they used a long block sander and did some fine-tuning to sand the surface. High-build primer really helps with this part! When the whole repair area was dead smooth and the repair completely invisible, they applied one final coat of primer on the surface to protect the area until it was ready to paint.