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Repairing Fiberglass Panels - Glass Act

CHP takes a look at repairing fiberglass body panels

John Nelson Jan 12, 2006

Mention "fiberglass" and "Chevy" in the same sentence, and most people immediately think "Corvette." It's a natural reaction, since fans of Chevy's sports car have been wrapping their, uh, persons in fiberglass for more than 50 years. On the other hand, Vettes certainly aren't the only Chevys running around with fiberglass body panels. Thanks to the aftermarket, thousands of other Bow Tie models run the streets--and the strip--wrapped in this lightweight composite material. All these 'glass hoods, fenders, bumpers, and front clips are at least as prone to battle damage as their metal kin, so we decided to look at what it takes to smooth over a fiberglass crackup.

The first item on the agenda, however, is deciding whether or not a panel should in fact be repaired. In most cases, this isn't an issue of whether or not the panel is repairable--it'll usually be pretty apparent if a panel has been damaged too badly for repairs. As with many things, to repair or not to repair comes down to the bottom line. Fiberglass is simply strands--or fibers--of glass that have been formed into mats, which are then bonded into structures. The structures favored by gearheads are lightweight and inexpensive, and therefore popular. The low cost of many 'glass panels makes repairing and repainting them a bad investment.

"You have to decide if repairing the part is cost effective," declared John Morrow, bodywork honcho at Johns Customz & Performance in Torrance, California. For instance, a typical fiberglass cowl-induction hood may cost $300-400, with bumpers and fenders running even less. A new part would cost less than the repairs, and there's still paint to pay for. On the other hand, a front clip can cost $800, and a "fancy" hood can approach this amount. At this point, repair becomes cost effective. Our guinea pig, an '84 Corvette, fell into this range. This Vette had an unfortunate encounter with the rearend of a Buick, leaving a hole in the hood. A used clamshell hood for a C4 Corvette can cost $900 (and quality may be iffy), while a new lid runs well over a grand. The cost of this repair, $400-500, made fixing the old a better investment than replacement.

Say you've come to the same conclusion--now what? The first tip Morrow gave us was on material--Johns Customz uses marine-quality fiberglass mat for the simple reason that it's designed to handle continual exposure to the elements. The second tip he gave us is to always work with clean material. This applies to both the new mat you're laying on and the old 'glass you're repairing. "The strands should be solid white," Morrow told us. "Fiberglass is like a sponge--it soaks things up." Once a panel is damaged, it's exposed to the elements, as well as dirt, oil, and other contaminants. The new fiberglass actually bonds with the old--picture the fingers of both hands interlaced. New strands intermingle with old. A repaired fiberglass panel should be just as strong as the original. For this reason, these impurities must be removed before new material can be laid on.

Speaking of fiberglass' sponge-like quality, the mix of resin and hardener used to bond in new fiberglass mat is critical. "The rate of hardening can varies according to the mix," Morrow told us. "But add too much hardener, and the material will crystallize, making it brittle and breakable." Be safe, we say, and follow the manufacturer's instructions until you've gained some experience working with these materials. Experience, of course, can make a job look easy, so we knew we were in good hands when Morrow turned us over to his top-notch 'glass man, Adrian Herrera. Herrera has been working with fiberglass since he was a child, building models and even a bicycle out of glass mat and resin. One of his specialties is creating custom body panels, so the prospect of repairing our Vette's broken nose didn't intimidate him in the least.

Herrera's skill at working in fiberglass proved impressive, so we weren't worried about the strength of his repairs. During the process, however, we did fret a bit about how the repairs would look. When the decision is made to repair a damaged 'glass panel, we all want more than a mere repair; we also want the part to look as good as new. Our expert bodyman achieved this as well. Skill and experience in working with fiberglass are crucial, but so is something of an artistic touch, the ability to form new material into an existing fender- or hoodline. We also replaced our subject Vette's tweaked urethane front bumper cover with a replacement from Mid America Motorworks, just to see how good the hood repair came out. As you'll see below, finding any of these repairs will be near impossible once the car is painted--and isn't that how it should be? Follow along, and we'll take some of the mystery out of the mysterious art of working with fiberglass.


When Corvette hood meets Buick bumper, the Vette's fiberglass hood is the loser. Luckily, it's cost effective to fix this piece. According to John Morrow of Johns Customz & Performance, we could bond the missing piece back in place--if we had it. Lacking that, this area will be rebuilt with new fiberglass.

While we had the 'glass mat and resin out, we decided to fix this small crack in the Vette's rear fender opening. Our fiberglass expert, Johns' Adrian Herrera, ground away the damaged area before we could even take a picture, exposing clean, solid white fiberglass. This is a must if the new fiberglass is to adhere properly.

The second order of business was to remove the fourth-gen Vette's clamshell hood and flip it over onto a pair of stands. There was a slightly more serious crack on the right fenderwell, one that went all the way through the panel. To make a strong repair, Adrian prepped both the top and underside of this area to receive new fiberglass.

An underside view of the Vette's hood damage reveals that this will be a complex job. Our '84-vintage subject feeds air to the engine through ducts in the hood, and the renegade Buick bumper broke into this area, which part of this ducting will have to be rebuilt. Adrian started by using a die-grinder on the shattered fiberglass in this area.

During the sanding and grinding prep on this hood, large chunks of 'glass came loose. This piece illustrates why so much material must be removed. It's ragged, and it's dirty, a bad combination if new fiberglass is to be applied. Check out the thrashed topside of the hood behind this piece--a lot more original fiberglass would be removed.

If you need any further proof that prep is crucial to a proper fiberglass repair job, look no further. Adrian used a mirror and sandpaper to get inside the Vette hood's air ducting, ensuring that he'd be working with fresh 'glass.

By the time Adrian was done grinding and sanding, the full extent of the damage to this hood was visible, and parts of the inner structure that must be rebuilt are easily seen. Most importantly, note the beveled edge that has been created around the repair area. This adds strength to the repair area by providing more area for the new 'glass to bond with the old.

Adrian fitted a piece of sheetmetal around the area to be repaired, the screwed it to the hood.

Once the screw lengths are ground off, this metal will provide a mold for the new fiberglass to come.

With the prepwork done, it was time to get down to laying some new 'glass. Johns Customz uses marine-quality fiberglass mat for greater durability against the elements. The fiberglass resin and its hardener should be mixed according to the manufacturer's specs. Mar-Glass is a body filler that contains strands of fiberglass--it actually bonds with fiberglass mat. The bottle in the middle contains a body glaze, a light filler that will be used to smooth out the repair areas.

After tearing the fiberglass mat to the desired size, Adrian used a paintbrush to soak each piece with resin before applying. This stuff is like a sponge, so plenty of resin should be applied during application.

Adrian worked quickly, sliding the soaked pieces of fiberglass mat under the hood's metal ducting and into the repair area outlined by the metal mold. Note the brush, which helps position the mat and keep it wet with resin. Adrian built this area up until the proper thickness was achieved, which took 4-6 layers of 'glass mat.

Adrian continued the hood rebuild with layers of fiberglass in the groove area created by the hood's leading edge and the outside of the hood air ducts. Adrian used a small piece of cardboard to fill in the broken area of the hood box-like area at the front of the hood. Again, soaked layers of 'glass mat are set down until the proper thickness is reached.

On the cracked quarter-panel, Adrian simply laid on a few layers of 'glass mat over the area he had ground clean. The ragged edge below the wheel opening was trimmed down with scissors.

The more serious crack on the front wheel opening section of the hood got layers of 'glass on both sides to give this area strength. This area is ready to work--as you might have guessed, small areas harden more quickly than large ones, like the hood.

While waiting for the initial hood repairs to harden, we turned our attention to the fender cracks. Using 200-grit sandpaper on a wheel, Adrian showed his expertise by working the wheelwell radius. It takes a practiced hand and great care to sand down the excess 'glass in this area without cutting into the fender line. Go slow and be careful if you try this yourself.


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