Fiberglass Repair - How to

I get alot of questions about composite repairs and the differences between working with fiberglass and carbon fiber. Many of the "wet layup" techniques with fiberglass apply to carbon fiber- this includes repair. Here's the text from an article I wrote for National Kart News in 2009. THis article was written specifically to hi powered racing go karts, but the techniques outlined in the article can be applied to most fiberglass or carbon fiber repair jobs.

(Here's a link to the article if you'd prefer to read it as it was published.
http://www.nkn.com/articles/SeatRepair0208.pdf)


Seats are one of the most abused parts on a kart
First, it gets numerous
holes drilled in it to locate
it in the chassis and find
that “sweet spot” to
make your kart handle
like it was intended.
Then more holes are
added to find the
proper rain setup
and yet another to
let water drain out
when it finally does
rain. Let’s drill some
more holes to mount lead
ballast, and maybe a few more to mount some of
the all important accessories for our shifter karts.
Next we’ll drive straight over that 4” tall curb
during a qualifier lap or maybe by “oops” during
a race. By mid season, your new seat is looking
pretty hammered and the scrapes on the bottom
of the seat are deep enough to see the track
surface below.
Fiberglass repair…yuck. It’s nasty, it smells
bad, and well, when done wrong can make your
expensive seat look like your hippy neighbor’s
home-built camper shell. Fear not. Fiberglass repair
doesn’t have to be a nasty job, you just need a
little instruction and some quality materials. Here’s
a step by step approach to help you do a quality
repair job with a lot less mess or hassle than you
anticipated. Along the way we’ll learn just enough
about composites to make you ready to tackle your
repair job with confidence.
Get organized
Make yourself a clean, clutter-free workspace.
Do not begin this task with your engine disassembled
on the workbench nearby. Fiberglass dust will find
its way into everything close by, so take a few
minutes to put away critical tools, instruments and
equipment. Lay out some newspaper on a clean
workbench surface. Tape the paper to the bench and
organize your supplies.
Assess the damage
Clean the seat with some lacquer thinner to
remove any grease, rubber marks, etc from the
damaged area. Tape off an area a few
inches larger than the damaged area with
masking tape. Begin sanding the damaged
area with 36 grit sandpaper, removing any
splinters completely. A die grinder with a
sanding pad works great for this but a little
elbow grease and some patience works
just as well. Don’t be concerned if you
sand all the way through the seat, it’s more
important to remove all of the damaged
material.
With the damaged area fully exposed
and the surrounding 2-3 inches scuffed
properly with 36 grit sandpaper, its time
for another round of cleaning. Wipe the
area thoroughly with a clean rag and
some lacquer thinner. If you’ve cut all the
way through the seat during the damage
removal process, flip the seat over so the
inside is facing you and place a few layers of
some two-inch wide masking tape over the hole.
Masking tape does a remarkable job of acting at a
temporary barrier that resin will not stick to when
cured. The tape will more or less take the shape
of the seat and will act as a temporary mold to
support the repair job.
Select proper materials
Here’s where most folks go wrong
with their repair job. They purchase
their fiberglass cloth at the home
improvement store simply because
it’s convenient. Convenient, yes, if you were
fixing Aunt Thelma’s bathtub on a Sunday
evening. But that’s not what we’re doing
here. We’re fixing a racing vehicle, so the
repair needs to be strong and lightweight. Do
yourself a big favor and purchase the right
stuff for the job. Among the most important
choices is the type and pattern of fabric. We
won’t get into engineering mechanics, but
here’s basically what you need to understand
about composites. Fibers need to be placed
in the same direction as the load that is
being placed upon it. Think about gluing 5
Popsicle sticks together side by side to make
a flat assembly. Breaking the assembly apart is
very easy along an axis perpendicular to the stick’s
glued edges. That’s because we’re only relying on
the strength of the glue to keep the assembly from
breaking. Now consider breaking the assembly along
an axis parallel to the stick’s edges. It’s considerably
more difficult (See illustration, page 55). Imagine
you were building an airplane wing from Popsicle
sticks.Which direction would you orient the sticks?
Now that we understand the importance of
aligning our material with how it will be loaded, it
should be pretty obvious that you want to avoid
the chopped strand matted material. This material
is easily identified by the tows of fabric going in
every direction with no apparent care for uniformity.
True, these types are less expensive than a woven
cloth, but they also do not offer the same strength
to weight ratio as a woven cloth. To make up for its
haphazard construction, the chopped strand mat
fabric must be considerably thicker, and therefore
heavier to obtain the same strength as a proper woven cloth. Save the chopped
strand mat for Aunt Thelma’s bathtub.
Instead, purchase a woven cloth worthy racecar or airplane construction.
Contrary to what you might think, you won’t have to spend a fortune either. You
should be able to purchase enough cloth to do several repair jobs for less than
$15. Aerospace grade fabrics come in a myriad of patterns, but for our project a
plain weave will work just fine. Plain weaves are just what they sound like, one
tow of vertical fabric weaves
above and below each passing
horizontal tow in a 90 degree
pattern. These types are used
most frequently for relatively
simple shapes or flat panels
and they are a great choice
for the bottom of our seat
because it will support the
load (of your body) uniformly.
Other weaves include twill,
crowfoot, satin, and leno,
wherein the patterns that skip
over and under corresponding
tows in a repetitive fashion.
These can be more flexible
than plain weaves and are good
for complex contours and tight
radius parts.
Aircraft Spruce and Specialty Co. supplies have a nice selection
of aerospace quality materials at a competitive price - even in small
quantities. Good deals can also be found on the web, but beware that many
web-based suppliers who offer seemingly good prices per square yard will
make you pay a premium for their “cutting” or shipping charges. Though
there are many good choices available from Aircraft Spruce, the Rutan bidirectional
fiberglass cloth is a great choice for its price, strength to weight
ratio, and easy of workability. It will also lay down nicely into a pretty tight
radius when laid up at 45 degrees to the woven pattern, so it can be very
versatile when required.
Prepare your materials
Cut up some fiberglass strips in varying sizes to fill in the void. The key to a
sound repair is to start with small strips barely big enough to fill the cavity and
gradually increase the size of the strips until you have the void completely covered
with maybe an inch or two larger than the damaged area. Starting with small
strips allows the glass to lie down nicely into the void, reducing the chances for air
entrapment during the lay-up. Air entrapment is the biggest enemy in composite
construction, so take your time to make strips that fit nicely into the void
and place them in order on your workbench from smallest to largest.
Now that you have your fiberglass strips neatly arranged, let’s go
over the basics of the resin system. Resins are typically comprised of
two separate parts, the liquid resin and a catalyst (hardener) that are
mixed together in proper ratios to form a solid. In composite construction,
the resin is merely the bonding agent that does little more than hold the
fiberglass cloth in its proper orientation. Resins have very little strength compared
to the fiberglass cloth, (Popsicle sticks example, below) so don’t get fooled into
thinking more resin will make a stronger part. Excess resin only adds weight to the
part and can make a real mess.
When selecting a resin system, consider the environment it will be used in,
temperature and humidity, time it takes to cure, mixing requirements, and cost.
There are many resin systems available. Most of us are familiar with the polyester
based systems. These are the inexpensive types found at hardware stores and
produce the familiar odor that we associate with boat repair shops. Polyester
systems are pretty thick (viscous), and don’t “wet out” the cloth as easy as some
of the epoxy based systems, so you end up with a heavier repair job. They can also
cure very quickly – sometimes in a matter of minutes making them a good choice
for performing emergency trackside repairs. Epoxy systems produce stronger
bonds than polyesters, and are designed for higher performance applications. We
selected the West Marine epoxy resin #105 and #206 hardener for our project
because it’s a high quality system that is aerospace proven and easy to use for
even a novice. The West System utilizes a mini pump kit that precisely meters the
proper ratio of resin and catalyst without the need for expensive scales. Epoxies
are much more sensitive to proper ratios than polyester resins so follow the
manufacturers’ mixing instructions carefully.
Regardless of the type of resin you use, keep in mind that once the resin
and catalyst agent are mixed, the mixture has a specified amount of “open time”
before it begins to cure. Ambient temperature can change the amount of open time
so be sure to follow the manufacturers instructions carefully. Once you’ve mixed
your resin, the clock starts ticking, so it’s important to have all your tools and
materials ready and organized ahead of time.
Do the lay-up
Having followed these easy steps, you are now ready to start the lay-up.
Disposable, lightweight, latex or vinyl gloves are an absolute necessity from this
point on. Meter out the proper ratios of your resin system in a plastic cup and mix
thoroughly with a wooden stir stick according to the manufacturer’s instructions.
Avoid stirring too quickly so as to “whip” air bubbles into the mixture; we’re not
making an omelet here. Stir the mixture uniformly while occasionally scraping any
resin off the sides of the cup. Also, it’s always preferable to mix up small amounts
or resin at a time and make new batches as needed. Mixing up one large batch of
resin that fills up the cup full will often cause an “exotherm,” a chemical reaction
that generates excessive heat and causes rapid, premature curing of the resin
system. Exotherms can ruin your lay-up, and can even be a fire hazard in extreme
cases. For small projects like this, a good rule of thumb is to only mix up batches
that are about 1/3 the cup’s capacity.
Using a small brush, apply enough resin into the cavity of the seat to lightly
cover the repair area. Begin laying your fiberglass strips into the cavity, starting
with the smallest piece. Press the glass into the cavity with a series of light
dabs from the brush. This technique is called “stippling” and is intended to push
the glass into the void while forcing out any trapped air. Make sure the glass is
uniformly covered with resin and that there are no white areas visible in the glass.
The glass should be completely translucent when properly wetted out. Continue
adding the layers of glass using the stippling effect on each layer. As you work your
way out with larger pieces of glass, you can begin using the squeegee to draw out
excess resin from the lay-up while forcing the glass down against the seat and
further removing any trapped air. Continue using the squeegee until you can no
longer see any “waves” of resin being pulled out in front of the squeegee.
Examine the repaired area for any remaining air bubbles or for any areas of
the glass that still appear white, indicating dry areas that will have to have some
additional resin added by stippling action. Allow the lay-up to fully cure according
to the resin manufacturer’s instructions. Avoid the temptation to poke at the layup.
Be patient. If you want to get an idea of the curing progress, check the leftover
resin inside the mixing cup.
Finish the repair
After the lay-up has fully cured, remove the masking tape and use your
sanding block to smooth the repaired area. There will likely be some very sharp
daggers of fiberglass out at the edges of the repair, so be careful handling the seat
until you have the area sanded smooth. Be certain to use a dust mask or respirator
while sanding. The sanded particles are very fine and can become airborne while
sanding, causing you to inhale them. Keep in mind that the seemingly harmless
fine particles are actually tiny shards of glass and resin. Start sanding the area
with course sandpaper to remove excess material quickly and work up to some
finer sandpaper to provide a nice smooth surface. Once sanded smooth to your
satisfaction, wash the seat with warm soapy water to remove any leftover sanding
dust. Finish the repair by applying a protective clear coat such as spray painted
lacquer, or enamel based clear. Clear coating the part will restore the discolored
repaired area to its original glossy finish to match the rest of the seat.
Fiberglass repair doesn’t have to be a real mess, take your time, use proper
materials and methods and you can take satisfaction in keeping your seat looking
and performing like new.
Sources:
Aircraft Spruce and Specialty, (877) 4-SPRUCE, www.aircraftspruce.com

Good luck!
Steve Frank
www.apexdesigns.net

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