We equate Corvette with fiberglass like we do pinstripes on a Yankees uniform or red on a can of Coca-Cola. It's just always been that way.
Fiberglass isn't the primary body material on Vettes these days, but the car continues a tradition of non-traditional, lightweight bodywork that began with the very first one off the assembly line in 1953. In fact, the reason conventional fiberglass isn't used any longer is because it's been replaced with better, lighter composite materials.
Beneath the skin, the Corvette has long been a pioneer in lightweight technology, from the use of aluminum wheels and suspension parts, to powertrain and chassis parts, and more. That heritage was advanced with the launch of the C5, which used materials including balsawood in the floor to minimize the car's overall mass; the C6 went several steps further, with the introduction of the Z06's aluminum chassis and the ZR1's extensive use of carbon-fiber body panels.
Importantly, the Corvette wasn't the first car to feature a fiberglass body, but it was the first mass-produced model. Similarly, the other lightweight components used in its construction led the way for the rest of the industry, slotting in between the ultra-expensive, low-production exotics and the high-production, value-priced family cars.
In the Beginning
Fiberglass was first considered for use on a GM vehicle by legendary designer Harley Earl. Besides a certain "exoticness" for the early '50s and the undeniable weight advantage, fiberglass offered an economical way to create the low-volume Corvette without investing in expensive sheetmetal-stamping dies.
GM's Parts Fabrication Operation was tasked with figuring out the intricate requirements for the '53 Corvette's body, while supplier Molded Fiber Glass Company (with some support from Owens Corning Fiberglass) got the contract to build them. Amazingly, the company was awarded the business in April 1953 and was asked to deliver the bodies by the June 1953 start-of-production deadline--and they had never built a car body previously.
Not surprisingly, there were plenty of unknowns when it came to designing and building those first Corvettes. Tests had to be done to determine how many layers of fiberglass were needed for the various areas of body, as well as the interior "tub." And for products so large, the correct ratio of resin and plastic hardener had to be determined, not to mention figuring out how long it would take for each hand-laid body to cure. Also, it wasn't clear at first how many separate parts would be required to construct each body, and a surface finish suitable for production-quality paint was far from assured.
The parts were produced with the "chop gun" method, in which pieces of fiberglass matting and polyester resin were "blown" into a mold to build up the part, layer by layer. With the benefit of nearly 60 years of hindsight, it's easy to criticize the quality of the early Corvettes' fiberglass bodies, but considering they represented the genesis of a production method that basically didn't exist previously, it was a feat that should be lauded.
The Introduction of SMC
Starting with the C3 generation in 1968, body parts were manufactured with a press-mold process, whereby the fiberglass material and resin were shaped in a die-like tool that produced smoother parts more quickly. It was a significant jump in forming technology and laid the groundwork for a change in the body panels' material in 1973. That year, the composition changed from conventional fiberglass to sheet-molded compound (SMC), which was composed of fiberglass, resin, and a catalyst formed under high heat and pressure. The ratio of resin to fiberglass was reduced with SMC, while the fiberglass itself was a bit coarser. The new material helped produce panels that were even smoother right out of the mold, meaning they required less surface finishing prior to painting. It also helped create a better final paint finish.
Technically, all Corvettes since 1973 have used SMC body panels, but the material composition has changed dramatically, featuring less traditional fiberglass and more lightweight plastic. The early SMC material created parts that were stronger and more rigid, but more brittle. As SMC technology and production experience evolved, Corvette engineers were able to alter the material composition and the body parts' specifications in order to trim the car's curb weight--an endeavor that was certainly welcomed though the later C3 years, as engine output kept eroding. Mostly, that meant making thinner body panels, because SMC was denser and stronger than conventional fiberglass.