Fleet Maintenance - 14

Best practices for
repairing composites
Hoke says technicians will largely be able to
rely on existing body service tools and techniques to make composite repairs. However,
there are some differences to be aware of, such
as cross-contamination.
"Whenever you put carbon fiber composites
directly against steel or aluminum, like if you bolted a steel or aluminum door handle to a carbon
fiber door panel, the carbon fiber will cause the
steel or aluminum to corrode," Hoke says. "It's a
galvanic corrosion problem. It doesn't hurt the
carbon fiber at all but will attack the steel or
aluminum. So, you need an insulator between
the two to prevent that corrosion."
Fiberglass, on the other hand, will not cause
that kind of galvanic corrosion. That is why Hoke
thinks fiberglass will ultimately emerge as the
preferred choice when it comes to composite materials in heavy duty trucks. Fiberglass is also fairly
straightforward for technicians to work with.
"You can typically drill holes in it with traditional tools," Hoke says. "That is not true with carbon
fiber where technicians need different [drill] bits."
Hoke believes the heavy duty truck industry has an advantage when it comes to
repairing composites.
"In the airplane world where we mostly use
carbon fibers, we see a lot of adhesively bonded
repairs," Hoke relates. "For instance, a puncture
or dent would be bonded, and then it would have
to be cured for a couple of hours at a fairly high
temperature of 180 to 350 degrees Fahrenheit. That
can be time-consuming and a little tricky to set up."
Conversely, in heavy duty trucks where Hoke
envisions more fiberglass composites, the repair
process could be much simpler.
"With fiberglass, a technician can consider
doing a 'bolted doubler' repair," Hoke says. "If a
hole is punched in the side of a trailer, for example,
the technician could cut another piece of fiberglass as an external doubler, drill a few holes,
and then bolt that doubler over the hole. It won't
necessarily look very pretty, but it would work."
Whether working with fiberglass or carbon
fiber, some in the vehicle collision repair business
lean toward a different approach to repairs.
"While CFRP (carbon fiber-reinforced plastic) is
a newer material, it still acts like FRP (fiberglass
reinforced plastic) and may, in fact, be repaired the
same way," says Doug Craig, Fusor field technical
support and training manager. Fusor, a brand of
Lord Corp., is a line of adhesives and coatings.
Craig says technicians can follow a standard
repair process.
"Technicians could use our 100EZ plastic
repair adhesive, which is a heat-set epoxy, along
with our cloth or any kind of glass or carbon fiber
cloth," Craig says.
The cloth is used to duplicate the amount of
fiber in the panel. The adhesive is used to hold
the cloth and entire repair area together.
For example, a rock is tossed into the air and puts
a hole in a fender. The technician should first taper
things out on the edge of that hole, followed by
building up the patch. The technician also wants
to have a lot of "grab" to the existing base material.

14 Fleet Maintenance | March 2020

1
»»Repairing a fender on a Peterbilt.
Properly support the backside of the
repair while "dishing out" the frontside
repair is critical so the repair material has
an adequate surface area to adhere to.
Photos courtesy of 3M's Automotive Aftermarket Division

Craig shares in detail how this can be done.
"You want one layer of backside reinforcement
that is larger than the hole," Craig says.
That reinforcement material then needs to be
cured. Then, the technician can start working on
the front side of the repair by applying layers of
adhesive cloth until enough has been built up to
duplicate the panel thickness.
"At this point, you start rolling it down to get the
air out of it," Craig explains. "Then we do another
heat [curing] to lock things up."
The heating steps are very important and should
be followed precisely by technicians. Epoxies will
generally harden in minutes or hours, depending on the product and certain environmental
factors. Additionally, depending on the epoxy, it
may continue curing for a period of time after the
heat source is removed.
"As it cures, the epoxy can change in size ever
so slightly," Craig says. "That's what can give you
ghosting or read-through. You still have a strong
repair, but you can see it. Some fleets might not
care, though."
If the fleet does care, technicians need to be
aware of this potential addition to curing time.
Even before this multi-step repair procedure
takes place, 3M's Spoto says one of the biggest
things technicians need to keep in mind is cleaning. Anytime a composite material is hit, it creates
a fracturing effect.
"That's why it is so important for technicians to
make sure the surface is clean before they begin
to sand," Spoto says.
It is also essential to keep the cleaning solvent
away from the fractured areas where fibers
are exposed.
"If the technician oversaturates the surface with
cleaning solvent, the solvent wicks down in," Spoto
explains. "When the technician completes the repair
and repaints it, it looks great. But later on, mapping
can set in."
Mapping refers to repairs where improper preparation results in the repair area showing through
the basecoat and clearcoat.
In addition to proper cleaning of the repair area,
Spoto says it's critical to provide proper support
to the repair area. He likens it to a broken arm,
where the doctor wants to support the structure
on all sides.
"When repairing a composite panel, you must
support the backside in order to give support to
the frontside," Spoto says. "When the backside is
properly supported, repairing the frontside is easy."
Another thing technicians need to think about
is the variety of materials they may run into simultaneously when making repairs. As Wurth USA's
Schiavoni reminds, technicians may run into a
heavy steel chassis and driveline, an aluminum
structural internal, and composite coatings. This
creates a situation where technicians may need to
adapt their normal practices, as well as the tools
they reach for in their toolboxes.
"With the inner structure, technicians in the
old days could spotweld all of the panels together when reassembling," Schiavoni says. "Now
you're having aluminum panels adhere to steel
inner panels and then a plastic panel over the top.
You can't weld aluminum to stainless steel; you
have to glue it and use self-piercing rivets which

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