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The use of “structural” adhesives to attach outer door skins, hoods, deck lids and body panels is nothing new. In fact, the vehicle manufacturers have been doing it for years.
Adhesives are used with both plastic and steel panels
in a variety of passenger-car and truck applications, so, consequently,
they must often be used during the collision repair process if
an OEM component that was glued in place needs to be replaced.
What’s new is the use of high-strength adhesives
as an alternative to spot welding or MIG welding for attaching
exterior steel body panels during collision repairs.
Why use an adhesive instead of welding? According
to the suppliers of adhesives, bonding offers numerous advantages:
- It saves labor time by reducing the amount of finish work
needed after installing a panel;
- It provides a better environmental seal, especially in critical
rust-prone areas, such as the joint between the fender and inner
- It won’t weaken heat-treated steel or alter the metallurgy
of the metal in any way;
- It won’t affect the corrosion protection offered by E-coated,
epoxy-primed or galvanized steel;
- It’s as strong or stronger than spot welding. Many adhesives
have shear strengths in the range of 3,000 to 4,000 pounds per
square inch (psi);
which concentrates stress at specific points;
quieter, stiffer seams. This, in turn, reduces metal fatigue and
the risk of cracking;
and materials are factored in;
steel, that cannot be welded together to be joined without the
use of special fasteners. The adhesive also creates a barrier
between the two metals, which helps to prevent electrolytic corrosion;
types of plastic without the use of fasteners; and
thinner-gauge sheet metal because bonding doesn’t create the panel
distortion or warpage that can result from welding.
In March 1996, several suppliers of structural adhesives participated
in a series of crash tests to find out if their adhesives could
meet the requirements for the Federal Motor Vehicle Safety Standard
(FMVSS) 301. The tests were conducted by MGA Research Corp., an
internationally recognized, independent test facility in Burlington,
The FMVSS 301 standard covers the integrity of the fuel system.
To pass the test, a vehicle’s fuel tank must remain intact and
not leak more than a few ounces of fuel following a 30-mph rear-end
collision. The test is conducted by ramming a stationary vehicle
from behind with a 4,000-pound mobile barrier.
For each of the participating adhesive manufacturers, three identical
vehicles were crash tested (different vehicle makes were used
by the various adhesive manufacturers). One car was stock. The
driver-side rear quarter panel and rear body panel were removed
from the other two test cars, then one car’s panels were reattached
by MIG welding, while the remaining car’s panels were glued in
place with adhesive only.
All vehicles, including the ones with the panels held by adhesive
alone, successfully passed the FMVSS 301 crash tests. The panel
seams that had been glued in place held up just as well as the
ones that had been MIG welded. The test also revealed that the
use of adhesive didn’t reduce the quarter panel’s ability to deform
and dissipate the impact forces created by the crash.
The FMVSS 301 crash tests proved that adhesives do have the strength
necessary for automotive metal-bonding applications and can be
safely and successfully used as a repair alternative to welding
in many applications.
In January 1997, further proof of the strength of adhesives was
obtained by Lord Corporation when a vehicle repaired with its
adhesive successfully passed an FMVSS 208 test, which tests the
integrity of the roof in a rollover crash. For this test, the
vehicle (a Nissan Sentra with a bonded roof panel) was mounted
at a 23-degree angle with the passenger side toward the ground.
The vehicle was then towed down a test track at 32 mph and catapulted
over to simulate a rollover. The test showed that the adhesive-bonded
roof maintained its integrity, and it confirmed the effectiveness
of the roof-skin bonding technique that Lord had developed.
But don’t sell your welding equipment just yet. Though structural
adhesives have proven their ability to hold various kinds of panels
in place during FMVSS crash tests, they’re still limited to certain
kinds of applications and aren’t a substitute for all kinds of
welding. No adhesive manufacturer currently recommends using a
structural adhesive on structural body components, such as frame
rails, subframes or rocker panels.
"We don’t like to refer to these products as structural adhesives
because we feel the term is misleading," says Tom Bray, of
3M Automotive Trades Division. "When you’re talking about
structural components in automotive applications, you’re talking
frame rails, posts and rocker panels. We do not recommend using
outer-panel adhesive on such components."
Bray says the term "structural" applies to the strength
properties of the adhesive only. "One definition of structural
means at least 1,000 pounds per square inch of shear overlap strength,"
he says. "A lot of adhesives have three to four times that
much. But another definition of structural is anything that’s
strong enough to cause substrate failure. Using this definition,
Elmer’s glue on wood is a structural adhesive because wood will
break before the glue pops away.
"If you bond an SMC panel onto a Chevrolet front-wheel-drive
minivan, you only need about 400 psi of shear strength before
the SMC breaks. So a structural adhesive that’s designed for this
type of application would not be the same as a structural adhesive
formulated for a higher strength metal-bonding application."
It’s important that body shop technicians don’t get confused about
structural adhesives, says Bray. Techs should not use them to
bond structural components on a vehicle because the adhesives
are not approved for this type of use; they’re strictly for exterior
sheet-metal panels only.
Bray says that although General Motors says its plastic quarter
panels on its first generation FWD minivans are structural, they
only add structural rigidity to the vehicle. They’re not load-bearing
components like the frame rails and rockers. The same applies
to exterior sheet-metal panels on most unibodies. The truly structural
members are clearly identified in collision repair manuals.
"The best advice we can give is this: If a technician doesn’t
know if a component is structural or not, we recommend welding
it rather than trying to bond it," he says.
Bray also says 3M is the only adhesive manufacturer that recommends
welding the seam between the rear body panel and quarter panel
rather than bonding it. "Our concern is not strength, but
that seam separation might occur in this area in certain kinds
of rear collisions," he says. "If the damage is not
noticed when the vehicle is repaired, there’s a possibility the
parts might peel loose from one another.
"Another recommendation we make is that when doing a beltline
cut on a quarter [not replacing the full quarter], the panels
should be welded rather than bonded. This will prevent the line
from showing and will give you
a better cosmetic repair."
Metal-bonding adhesives are available from various suppliers and
come in three basic categories: acrylics, epoxies or urethanes.
Acrylics – Lord Corporation’s Fusor Metal Bonding Adhesive
is a two-part acrylic. This type of adhesive has a working time
of about 45 minutes and sets up in one-and-a-half to two hours.
At that point, the adhesive is hard enough so the clamps can be
removed and the technician can continue finishing. Full cure is
in eight hours. Heat is not recommended with this type of adhesive
to speed curing.
Epoxies – Dominion Sure Seal’s Epoxy Door Skin Adhesive,
Kent’s Uniweld Door Skin Adhesive, SEM Product’s Nonsag Door Skin
Adhesive and 3M’s Panel Bonding Adhesive are all two-part epoxies.
Working times and curing times vary with the product. Kent and
SEM have working times of about 30 minutes, with set up in one
hour and full cure in 24 hours at 70 degrees F. 3M also has a
working time of about 30 minutes but recommends that clamps be
left in place for four hours before proceeding with work. Full
cure is also in 24 hours. Dominion’s working time is about two
hours, with a six-hour setup time and 24 hours for full cure.
The curing time with all these products can be greatly decreased
by applying heat. Heat lamps at 120 to 140 degrees can reduce
curing time to about an hour.
Urethanes – Polymer Engineering Corporation’s Duramix
Medium Metal Panel Door Skin Adhesive is a two-part urethane.
Duramix has a working time of 45 minutes, with a six-hour setup
time and 24 hours for full cure. Heat can also be used with this
product to reduce cure times. For every 20-degree increase in
temperature over 75 degrees F, the cure time will be cut in half.
It’s important to note that all adhesives must be kept warm for
curing to occur. When ambient temperatures drop below about 50
degrees, curing stops. For this reason, vehicles should be kept
indoors in a heated area until the adhesive has time to cure.
Technicians need to be aware of the differences in adhesives because
each type of product is unique. What’s more, application instructions
vary from one adhesive manufacturer to another, so you should
always follow the specific application and usage recommendations
by the product’s manufacturer.
For example, Tom Oliver of SEM says its Nonsag Door Skin Adhesive
should be used to bond door skins only. The product has a shear
strength of 2400 psi, which is fine for door skins but not for
other metal-bonding applications. Oliver says SEM is currently
developing a new adhesive for quarter panels and roofs.
Jim Perritt of Lord Corporation says the nice thing about using
adhesives to bond metal is that it’s hard to screw up. "Unlike
welding, it takes no special skills, " says Perritt. "But
you do have to use the product correctly."
Perritt says surface preparation is the key to getting a strong,
long-lasting bond. "You do essentially the same kind of surface
preparation that you do for welding, except that you should also
grind the primer off."
Most other adhesive manufacturers agree on this point. Though
the E-coating on new sheet metal provides corrosion protection,
the quality of E-coats can vary (some may not adhere as well as
others, say the adhesive manufacturers). What’s more, E-coatings
can sometimes separate from a panel at 20 to 30 degrees F below
zero, which may cause a bonded seam to pull loose. That’s why
the adhesive manufacturers recommend removing all paint, primer
and E-coat prior to bonding.
What about corrosion? Adhesive manufacturers say that as long
as all of the exposed metal is covered with adhesive, there should
be no concerns about corrosion. A squeegee can be used to spread
the adhesive over the exposed metal on both mating surfaces to
assure complete coverage prior to bonding, and a small bead of
adhesive can then be applied to the panel to glue it in place.
When replacing a quarter panel, the bulk of the damaged panel
can be cut away with an air saw or chisel. The mating flanges
should not be cut; instead, they should be removed by drilling
out the spot welds with a spot weld cutter. The remaining edges
can then be chiseled away.
Once the panel has been removed, any damage along the mating edges
should be straightened using a hammer and dolly. The mating edges
on the body, along with the entire outer edge of the new panel,
should be ground down to bare metal so the adhesive will bond
to the metal itself and not to the paint or primer (which might
After trial fitting the parts to make sure they align properly,
a 1/4- to 3/8-inch bead of adhesive can be applied along the prepared
edge of the new panel. A small amount of adhesive should be squirted
out of the applicator before applying the sealer to the panel
to make sure the adhesive is mixing properly. Once the adhesive
is applied, the panel can be positioned on the vehicle and clamped
If a panel needs to be readjusted, it should never be pulled loose
– doing so can introduce air bubbles that might weaken the bond.
If the panel needs to be adjusted, it should be slid to maintain
Panels should be clamped evenly all the way around to provide
proper support; but don’t clamp the panel too tightly because
doing so can squeeze the adhesive too thin and prevent proper
curing and bonding. Jim Bidwell of Polymer Engineering recommends
a minimum thickness of 5 to 20 mils with its urethane-based product,
while Mike Medrek of Kent recommends a minimum thickness of 1/16
inch with its epoxy-based product.
If you’re skeptical about using an adhesive to install a quarter
panel, one adhesive manufacturer suggests using adhesive along
the wheel-well area and dog leg where it comes down to meet the
back door, then welding the rest of the panel in place. Using
the adhesive here will give a better seal in an area that’s hard
to seal and minimize the risk of corrosion.
Replacing Door Skins and Roofs
Door skins can be removed by grinding away the edges, then removing
the outer panels. Damaged areas on the mating flange should then
be straightened with a hammer and dolly. The entire outer edge
of the door frame should be ground down to bare metal to remove
all paint and primer, and the inner edges of the new door skin
should be scuffed before prefitting the parts to check alignment.
A 1/8-inch bead of adhesive can then be applied to the door skin.
The door skin should be lightly clamped in place while rolling
the hem flanges. Allow one-and-a-half to two hours for the adhesive
to set, and then scuff the edges of the door skin with a scratch
pad and prime per the paint manufacturer’s instructions.
When replacing a roof panel, the E-coating should be removed in
the bond area (a red scratch pad is recommended). A 1/8-inch bead
of adhesive can then be applied to the new panel and clamped in
Removing the Adhesive
Making collision repairs on a vehicle that has bonded panels requires
the use of a heat gun to loosen the adhesive. Epoxies and acrylics
soften and lose their grip when heated to 400 degrees F (250 degrees
for urethane). A chisel can then be used to pry apart the mating
surfaces to peel the panel loose.
Adhesives are strong in the shear mode but weak in the peel mode,
so pulling panels loose isn’t that difficult once the material
is heated and starts to loosen.
Another technique that works is to use an air chisel, saw, cutoff
wheel or shears to cut away most of the damaged panel, leaving
about a one- or two-inch strip around the edge that has the glue
on it. The remaining strip can be pried loose and peeled off.
Once the panel has been removed, all traces of the old adhesive
must be removed by scraping or grinding. The mating surface then
needs to be straightened, ground down to bare metal and cleaned
prior to installing the replacement panel.
A Gripping Subject
Like other changes and advances made to the collision repair process,
high-strength adhesives are a subject worth investigating. The
use of these adhesives is a relatively new procedure – a procedure
that might prove beneficial to your business if you take the time
to learn how to use them properly.
Writer Larry Carley is a contributing editor to BodyShop Business.