And though there are plenty of other performance
cars and sports cars on the market, most people consider the Corvette
the "true American sports car."
The Corvette is unique because it was the
first, and for a long time, the only production car to have an
all-fiberglass body. Other vehicles have used fiberglass and plastic
composite bodies – including the Pontiac Fiero, GM front-wheel-drive
minivans, Saturns and the Dodge Viper. There also have been numerous
applications for polyester/fiberglass (PU) and sheet-molded compound
(SMC) for body panels and components; those applications include
the front and rear facias on many older GM models; the hood and
liftgate on Ford Aerostar minivans; Ford Bronco II liftgates;
the rear fenders on GM Sportside pickups; and the hood, fenders
and cabs for many over-the-road trucks. But the Corvette is the
only vehicle with a long history of using composites.
That history may be reveled in by the OEM
and car enthusiasts, but for the body shop technicians faced with
a damaged Corvette, it makes for unique repairs – ones your shop
should be familiar with.
Up to Speed
In 1973, Chevrolet changed the composition
of its polyester resin and fiberglass. This "second-generation"
material uses a lower ratio of resin to fiberglass, a coarser
grade of fiberglass and different filler materials. The change
made the body panels stronger and more rigid, but also more brittle.
What’s more, the kind of fiberglass resin and polyester body filler
used to repair the earlier Corvettes wouldn’t stick well to the
new material. To achieve good adhesion, up to 24 hours of cure
time was sometimes necessary unless heat was used to speed up
the process. Even then, the resin or filler might flake loose,
so new resins and body fillers that were compatible with the new
fiberglass were introduced.
The current generation of Corvettes is also
different from the earlier models because it uses SMC body panels
(which are similar to those on the Fiero and GM front-wheel-drive
minivans). Though SMC is made of resin and fiberglass, it’s manufactured
in a unique way and contains a different mix of ingredients.
Conventional fiberglass panels that are hand
laid or sprayed in one-sided molds typically contain about 70
percent polyester resin and 30 percent chopped glass fiber. This
type of manufacturing method is relatively simple and is still
used by some kit-car and aftermarket manufacturers, but the technique
doesn’t produce panels with consistent thickness. This type of
construction can be identified by examining the underside of the
panel. If you see a rough finish (strands of fiberglass or matting),
it’s hand-laid or sprayed conventional fiberglass-reinforced polyester.
SMC, by comparison, contains about 40 percent
resin, which may be polyester, epoxy, vinyl ester, styrene or
a blend of each; 20 percent chopped-glass fiber; 33 percent calcium-carbonate
filler; and 7 percent resins and hardeners that improve the surface
finish of the panels. SMC is made by sandwiching glass fibers
between two sheets of resin, which are then pressed together and
heat cured in a compression mold to form the finished panel. The
result is a smooth finish on both sides and a more consistent
thickness. SMC is denser and stronger than ordinary fiberglass,
but differences in its chemical makeup require compatible repair
materials.
Material Background
Polyester/fiberglass and SMC are both thermoset
plastics, which means once the material has cured, it cannot be
softened or welded with heat like thermoplastics can. If a panel
is cracked, ripped, torn or punctured, the damage will have to
be repaired chemically, using fillers or adhesive.
Older Corvettes with fiberglass-reinforced
polyester body panels can generally be fixed using traditional
fiberglass-repair techniques. Spot repairs can be made using fiberglass
resin or ordinary polyester body filler, and woven, mat or chopped
fiberglass can be used for filling and reinforcement. Damaged
fiberglass panels can also be patched, sectioned or replaced as
needed, depending on the extent of the damage. There are numerous
aftermarket sources for reproduction Corvette hoods, fenders,
front ends, rear ends, quarter panels, repair panels and other
stock, as well as customized body parts.
SMC panels on 1984 and newer Corvettes can
be repaired just as easily, but they require repair materials
that are compatible with the different chemical makeup of SMC.
General Motors
doesn’t recommend using fiberglass resin for
bonding or cosmetic filling on SMC. Instead, the approved repair
materials recommended by GM are GM Goodwrench Structural Bonding
Epoxy (p/n 12345726), Lord Fusor SMC body-panel repair adhesive
or an equivalent product.
Minor surface scratches and gouges in fiberglass
can usually be filled with resin or ordinary polyester body filler,
while those in SMC panels can be filled with the GM structural
epoxy, Fusor body-repair adhesive or other similar products. Tears
and holes, on the other hand, usually require patching and/or
filling and reinforcing with fiberglass cloth, matting or scrap.
The Repair Process
The basic repair procedure for patching a
hole is to cut away the damaged material and sand the paint back
to a feather edge adjacent to the repair area. Fiberglass and
SMC often have tiny spider-web or hairline fractures that radiate
out from damaged areas or stress points. To find the full extent
of such cracks, press or pull on the damaged area to flex the
panel. Because cracks tend to keep on growing, the cracks must
be either eliminated by removing the damaged section or drilled
to stop them. Drilling a small 1/8-inch hole in the end of a crack
will usually stop it from growing, and the hole can then be filled
and the rest of the crack ground out and filled to repair the
damage.
With more extensive damage, cut or trim away
any loose material using shears or snips. Sections or panels can
be removed using a reciprocating saw or cutoff wheel. Remember
that cuts should be made along existing seams when panels are
being replaced.
Both surfaces, inside and outside, must be
thoroughly cleaned to assure good adhesion of any repair material.
Remove any undercoating from the backside of the panel or damaged
area, then wash the area with soap and water to remove dirt or
any mold-release agent. This is especially important if you’re
replacing panels because mold release (usually silicone) acts
like grease and prevents resin, epoxy and adhesive from forming
a good bond. A steel-wool pad or similar product will usually
remove any residual mold release from the panel. The damaged area
or panel should also be wiped with a solvent that does not leave
a residue – such as VMP or a high-flash naptha – to remove any
grease or wax. Use the solvent sparingly because it can soak into
the fiberglass and migrate out, later marring the finish.
Patching Panels
If a hole is being filled, a backing patch
can be made with fiberglass cloth, mat or even a piece of discarded
material that’s the same as the panel itself. A backing is recommended
for maximum strength. Even so, some Corvette owners (especially
owners of older Vettes) may not want a repair that can be seen
even from the inside.
Normally, both sides of the damaged area would
be feathered. A patch would then be applied to the inside first,
then the damaged area would be filled and leveled using repair
material and fiberglass cloth, mat or hair.
To do an "invisible" repair, skip
the patch and proceed as follows:
- Cut and trim away any torn, jagged or loose material, then
feather edge only the outer edges of the damaged area. - Cut a piece of wax paper or plastic food wrap that’s big enough
to overlap the damaged area by 2 to 3 inches on all sides, and
carefully tape it to the underside of the damaged area. Make sure
the backing is firmly attached to the panel and doesn’t protrude
or sag. For customers who are really picky, you can make a "reverse
mold" to serve as a temporary backing for the area to be
filled. The mold will simulate the surface texture of the underside
of the original panel so your repair will be nearly undetectable.
Such a mold can be made by coating the underside of an adjacent
area (or the same panel from another Corvette) with release agent,
then applying a couple layers of fiberglass and filler material
as though you were making a patch (modeling clay that hardens
will also work). When the "patch" hardens, peel it loose,
spray it with more mold release and reposition it under the area
to be filled. The resin will then fill the crevices in the mold
as the damaged area is filled, duplicating the original texture
of the underside of the panel. - Mix up your repair material (resin for fiberglass, epoxy or
adhesive for SMC) and apply a layer to the damaged area. Then
overlay or fill with fiberglass cloth, mat or hair. - Apply more repair material (resin, epoxy or adhesive) so it
soaks into and thoroughly saturates the fiberglass. Cover with
a second sheet of wax paper or plastic food wrap, and use a rubber
roller or squeegee to massage the material into the fiberglass
and work out any air bubbles. - Add successive layers of fiberglass and filler material as
needed to build up and fill the damaged area until it’s flush
with the surrounding surface. - Curing for an hour at 150 to 250 degrees with an infrared
heat lamp placed about 30 inches from the work surface is recommended
to prevent the repaired area from shrinking. - Finish as usual by smoothing the surface with a No. 180-grit
disk or sandpaper, followed by No. 220 to No. 320 wet or dry sandpaper
on a block.
Finish It Off
As a rule, fiberglass and SMC can be painted using conventional
refinishing techniques. If the old paint needs to be stripped,
plastic blasting media or water is recommended. Sandblasting may
be too aggressive on plastic, and chemical strippers may cause
damage.
Some Corvette experts warn against using self-etching primers
and lacquer primer on fiberglass or SMC. Instead, they recommend
using a two-part epoxy or polyester primer.
One thing all Corvette experts – and anyone who’s repaired a damaged
fiberglass quarter panel – agree upon is that repairs to this
sports car require special technical skills as well as safe material
handling. With that knowledge, your shop can promise owners quality
repairs and many more Corvette summers.
Larry Carley is a contributing editor to BodyShop Business.
Safe Handling
If you’re itching to work on a Corvette, take care to protect
yourself against an itch or worse. Despite its soft appearance,
glass fiber isn’t something you want to handle with your bare
hands. Tiny slivers of glass can become embedded in your hands
or in any exposed flesh, causing an itch, rash or allergic reaction.
Gloves and long-sleeve shirts should be worn at all times. Another
alternative is to apply a protective cream to your hands and skin
to shield yourself against the fibers. Washing afterwards with
cold water can also help minimize skin irritation.
Likewise, grinding and sanding fiberglass creates dust that contains
tiny shards of glass fiber. These, too, can be very irritating
– not only to your skin, but also to your nose, throat and lungs.
A dust mask or respirator should be worn when working with any
fiberglass panels.
To protect your fellow workers, use a vacuum attachment on cutting
and sanding tools for dust control. Also, if possible, work in
an area that’s separated from the rest of the shop by some kind
of dust barrier – walls or plastic curtains. Enclosed areas must
also be properly ventilated because resins can produce toxic fumes.
Words of Experience
D & M Corvette in Downer’s Grove, Ill., has had the itch for
Corvettes for the past 12 years. Specializing in Corvette sales,
service and restorations, the shop performs mostly restoration
work along with some collision repair.
"Many Corvette owners won’t accept a patch job and insist
on replacing an entire panel or section," says shop manager
Dan Sobieski. "We buy many of our panels directly from General
Motors, but for parts that are no longer available, we’ll use
aftermarket parts." Sobieski says the quality of most reproduction
parts is pretty good, and the cost is often the same or more than
the OEM part.
The toughest jobs, says Sobieski, are the ones that require replacing
the upper or rear surround panels because there’s so much disassembly
and reassembly involved.
"It’s also tough to find experienced bodymen who will do
this kind of work. The itch problem turns off a lot of bodymen
to working on fiberglass," he says.
Jeremy Dickerson, one of D & M’s bodymen, says working with
fiberglass is no big deal. "I think fiberglass is a lot easier
to work on than metal because you can build up surfaces to compensate
for things," he says.
The key to replacing fenders, doors, quarter panels and hoods,
says Dickerson, is to make sure everything lines up, the symmetry
is maintained and all the seams are correct. For instance, doors
should have about a 1/8-inch gap all the way around. And remember,
fiberglass panels won’t give like steel panels, so any structural
repairs or straightening that has to be done must be done before
the body panels are attached to the frame or supports.
Selling the American Sports Car
When Corvettes were first introduced back in 1953, their fiberglass
bodies were quite a novelty. All ’53 models were white with red
interior and were powered by a 150-horsepower "Blue Flame"
six-cylinder engine mated to a two-speed Powerglide automatic
transmission. Because Chevrolet only built 300 that first year,
it’s doubtful you’ll ever see one of these classics in your shop
unless you’re doing serious restoration work.
In 1954, production was moved from Flint, Mich., to St. Louis.
During the "classic" years (1953 to 1962), Chevrolet
produced a total of 69,015 Corvettes – which isn’t many by today’s
standards. In 1963, sales nearly doubled when Chevrolet introduced
the new Stingray body style. Split-window fastback, sleek styling
and hide-away headlights gave the car an aggressive, new look.
The split-window disappeared in ’64 to improve rear visibility,
but the basic body style remained essentially unchanged through
1967. Sales during this period totaled 117,964.
In 1968, a new car based on Chevrolet’s Mako Shark show car was
introduced. The new body style was longer, wider and more curved
than its predecessor, and it continued with ongoing refinements
through 1982. The Stingray nameplate was temporarily discontinued
for one year (1968) but reappeared on 1969 through 1976 models.
Sales during this period were more than half a million (542,861)
cars, with sales reaching an all-time high of 53,807 in 1979.
There were no 1983 model-year Corvettes officially produced for
sale to the public – though if you ever get a chance to tour the
Bowling Green, Ky., Corvette plant, you’ll see one on display.
The all-new Corvette that was introduced in 1983 was marketed
as a 1984 model-year car, and sales that year were the second
highest in Corvette history (51,547). The same basic body style
has continued through the 1996 model year with various facelifts
and will be replaced with an all-new body style in 1997.
