Composites have numerous advantages over conventional building
materials, including ease and durability of repair. However, damaged parts
are often replaced simply because many individuals lack the knowledge to
repair them or are intimidated by the prospect of fixing them. This
brochure describes in detail the modern repair techniques necessary to
restore the function to damaged composite parts. It is divided into two
sections, so both structural and cosmetic repairs can be covered
The techniques described will work for any fiberglass structure and
many "advanced" composites of Kevlar or carbon fiber. Please
realize that specialized procedures beyond those described here may be
necessary with certain parts. Manufacturers of structural parts often
publish detailed repair information specific to their parts. The
manufacturer’s instructions supersede all other information and should
be followed completely. Most of the time, the information is more specific
and does not contradict the standard repair procedures as outlined here.
When the suggestions in this pamphlet are executed well, strong repairs
Once a few fundamentals are understood about repairing composites, most
projects can be completed successfully and a long life span can be
expected. We will begin by listing and explaining three main principles of
1) All repairs are secondary bonds, so they rely upon the adhesive
quality of the resin for their strength.
Structural repair theory begins by recognizing the difference between a
repair and the original piece. When a part is first fabricated, all the
resin in it cures chemically as a single unit regardless of the number or
orientation of the reinforcement plies. This is called the primary
structure or bond, and it is the strongest form in which a part can exist.
Once the part is damaged, all repairs become secondary bonds attached to
the original primary structure. In real life, this means that the repair
is only as strong as the adhesive used to make it. For this reason,
never use a weaker resin than what the part was made with originally. In
fact, stronger adhesive resins are sometimes used for repairs. However,
even this substitution must be cautiously tested so it does not change the
performance of the part.
2) Increasing the surface area will increase the strength and the
durability of the repair.
Since repairs depend upon adhesion to the primary structure, increasing
the surface area of the bond line will increase the strength and longevity
of the bond. This is usually done by taper or scarf sanding the area next
to the damage so the void can be filled gradually. The size of the taper
is expressed as a ratio comparing the depth of the repair to the width of
the taper. Generally, the stronger or more critical the repair needs to
be, the larger the taper ratio.
An alternate method of increasing the bond surface is step sanding.
This procedure defines the size of the inner repair, then removed ˝ inch
per ply of the surrounding material while working toward the part surface.
The repaired surface grows considerably and the fiber orientation is quite
evident in each step.
3) Strive to duplicate the thickness, density, and ply orientation of
the original laminate to maintain the functionality of the part.
Many folks go overboard on repairs thinking that if a little bit is
good, then more is even better. That is dangerous thinking with reinforced
composites, because as a part becomes thicker, it automatically becomes
stiffer, regardless of the material in use. The proper approach is to
carefully replace every ply that has been removed while preparing the
damaged area with an identical material in the same orientation. This
ply-for-ply replacement approach will guarantee the structure can
withstand the same loads as the original.
These three principles are the basis for understanding composite
repair. The complete sequence is explained below, but comprehension of
these points will help you understand how the steps are interrelated. The
sequence is straight-forward and extra explanation has been added for more
detail at key stages. However, it is difficult to cover examples of every
problem in this small brochure format.
Structural Repair Sequence
1) Inspect the damage, identify the materials involved, and determine
whether the part should be replaced or repaired.
Damage generally falls into four categories, but they can all be
present if the impact was severe enough. Tears, holes or punctures,
crushed cores, and delamination are the most common problems. To make
reference quicker, use a thick pencil to outline the boundary of the
damage. Close inspection is necessary as the problem area often extends
farther than can be easily detected visually. A coin tap test is an
effective way to test the surrounding area. An audible difference is easy
to detect between solid laminate and crushed material when tapped with the
edge of a coin.
Identification of the materials involved, especially the resin, can be
more difficult. Begin by determining the type of fiber reinforcement used.
This will often give clues as to the type of resin employed. Marine
applications where chopped mat is most prevalent probably used general
purpose polyester resin. Fabric based auto and industrial parts are
generally made with either polyester or epoxy resin, so epoxy would be the
safe repair choice. Carbon fiber and Kevlar parts can be repaired with
either isophthalic or epoxy resin. Consult the original manufacturer
before attempting any repair to an aircraft. They can recommend the exact
system to use.
A new composite material is gaining widespread use in the automotive,
industrial and personal watercraft markets that presents unique repair
problems. It is called SMC, or Sheet Molded Compound. Recognizing SMC from
other types of FRP composites is critical so the proper repair can be
performed. Parts made with SMC are produced in compression molds, so they
are smooth on both the inside and outside. That is the first clue to look
for when identifying them. Next, SMC parts do not have an outer gel coat,
but they are usually painted or color molded. When the paint is sanded
off, the underlying surface has a marble appearance. Finally, when damaged
SMC is sanded, short coarse fibers are exposed and a dryer powdery dust is
produced compared to conventional materials. These hints will make SMC
identification quite straightforward.
SMC is a polyester-based material, but it cannot be repaired with
polyester resin. This is due to the mold release agent that is present
throughout the entire SMC part. Unlike conventionally molded parts where
release agents are applied to the mold surface, SMC is compounded with
them in the resin mix for quicker processing. This means that as the
damage is sanded to prepare a good bonding surface, fresh mold release
agent is exposed. Polyester resin products are not strong enough to adhere
to this surface. SMC SHOULD ONLY BE REPAIRED USING EPOXY-BASED RESINS,
FILLERS AND ADHESIVES. When painting, use only catalyzed type paint
Once the extent of the damage and the type of material used is known,
determine if the part should be repaired or replaced. If the manufacturer’s
specifications are available, check whether the damaged area is too large
to be repaired. If no information can be reviewed, make a quick estimate
of the materials and labour time needed for the repair and compare the
figure to the price of a new part. A savings of at least 50 percent is the
typical cut-off point to warrant proceeding with the repair.
2) Remove the damaged material and prepare the area for bonding.
Try to identify the sequence and the orientation of the material as it is
removed layer by layer.
When a part is broken or crushed it is difficult to realign the pieces
with each other because the frayed fibers tend to "hang up" on
one another. Use a saw blade to cut the length of cracks or tears. This
relieves the stress on solid laminates, which often return to their
original shape with little force.
Cored parts tend to pancake and mushroom, further complicating
realignment. A router is excellent for removing damaged core material
without disturbing intact face skins. Try to remove as little material as
possible so the repair does not grow too large. However, solid laminate
must be exposed for a good repair. Continue coin tapping and grinding
until all the damaged material is removed. Finally, determine the density
of the core so it can be replaced with the same material.
Next, support the part so nothing gets distorted during the repair
process. This can be as simple as strips of 2 inch wide masking tape, or
as elaborate as a custom-made clamping fixture. Generally,
high-performance parts require very precise support systems.
With the part supported, proceed to preparing the bonding surface by
grinding a taper or steps around the damage. This is a critical step for
functional repairs, but it is also the most overlooked and abused. If a
taper is to be used, measure the depth of the valley and calculate how far
the sanding must extend to achieve the desired ratio. Mark the outer edge
of the taper using a pencil, and begin sanding inward toward the valley.
Be sure to remove material slowly so the taper progresses evenly. Write
down the orientation and type of fabric used as each layer becomes exposed
so it can be replaced in the same way.
When step sanding, the initial calculation becomes more precise.
Assuming the damage is circular, 2 inches in diameter and there are 5
plies in the laminate, make concentric circles expanding 1/2 inch per ply
from the edge of the innermost circle. The final diameter of the prepared
area will be 7 inches.
Begin sanding in the center until the deepest layer is exposed. Step
out 1/2 inch and sand down to the second deepest layer and so on, until
all five steps are prepared. A right angle grinder offers the best feel
and control for this delicate procedure.
For personal safety and cleanliness, tape the hose of a shop vacuum to
the work surface so dust can be removed while grinding is taking place.
When the sanding is complete the whole surface needs to be thoroughly
cleaned. Vacuum any remaining dust and then wipe the surface with a
solvent rag. Acetone is usually sufficient for removing oils, greases,
dust, waxes or other contaminants which would interfere with repair
3) Laminate the repair patch.
Begin this procedure by precutting the core replacement and the
reinforcement plies that will fill the repair taper. Cut the core first to
fill the deepest hole. Some cores like Nomex honeycomb have their own
orientation which needs to be aligned in the part. Be sure to check this.
Consult the list that was made during the preparation process so that each
reinforcement ply is cut to the proper orientation. Cut each ply so it
fits precisely into the step that was prepared for it. Modern repairs are
made ply-by-ply, so the smallest piece is intended to go into the bottom
of the valley. Stack the reinforcements near where they will be used, with
the first layer to be placed in the bonding area on top of the pile.
Mix the appropriate resin or adhesive system for the repair patch.
Pre-weight the reinforcement schedule and mix only the same weight of
resin. This will keep the resin content within a reasonable 50:50 ratio.
Pre-wet the entire bonding area with resin, then begin saturating each
ply of reinforcement before it is placed into its step. Work on a sheet of
plastic so the fabric can be easily lifted from the table once it is
ready. A flexible rubber squeegee is the best tool for spreading the resin
evenly through the fabric and removing excess which may be present. Place
the reinforcement into its spot on the repair ensuring the proper
orientation. Stop every few layers to compact the patch as much as
possible. A squeegee or grooved roller works well for this. Continue
stacking the repair plies until all the fabric that was removed has been
replaced. Never exceed one layer at a time. A final cover layer is then
added over the entire area.
Plan on compacting the final repair patch as densely as possible while
the resin is curing. Vacuum bagging is the most uniform method, but
squeegees, rollers, or other clamping pressure will work adequately.
Follow the manufacturer’s recommendations for curing the resin or
adhesive. If a two-sided repair has been planned, preparation of the
opposite side can begin once the first patch has cured.
4) Inspect the repair before putting the part back into service.
Use the coin tap method to inspect the cured repair. The entire
structure should resonate the same solid sound. Non-destructive load
testing can also be used for greater confidence. This consists of
stressing the part up to its expected service limit but not beyond. If the
part fails prematurely, it should be discarded. For more critical
structures, a testing laboratory or specialized equipment may be necessary
before sign-off. If all is well, proceed to the cosmetic repair sequence.
Cosmetic Repair Sequence
A part’s outer cosmetic finish is designed to hide and protect the
structural reinforcement below, as well as be aesthetically pleasing. It
is this surface by which most people will judge the whole structure. Even
slight damage like scratches and gouges will mar the finished appearance.
Worse, these often create a direct path into the structural layers,
causing even larger problems. Cosmetic blisters are a problem which, left
untreated, can turn entire boat hulls into sponges.
The cosmetic repair sequence is similar to the structural sequence, but
filler or gel coat replaces any reinforcing material. Surface preparation
is still the most important aspect of a long-lasting repair. Finish
sanding and polishing are the only differences, but they make a good job
look truly professional. Many fear tackling cosmetic work because they
think it requires expensive spray equipment for satisfactory results.
Equipment is important, but proper material selection will give better
1) Inspect the damage and remove any other loose or unsupported surface
Inspection of cosmetic damage is just as important as it is with
structural repairs. Gouges often leave undercut areas of unsupported gel
coat which at first appear fine. Press on them with a blunt tool and they
will easily crack away. Every such area must be identified so it can be
prepared correctly in the next procedure. Use a marker to circle each spot
that needs prep work. This is especially important on large blister repair
jobs. Once the dust starts to fly, it isn’t always easy to see the
2) Prepare the surface for the addition of gel coat or filler.
Begin by taping off the area surrounding the damage. This will keep
sanding scratches around the damaged area to a minimum. Next, wipe the
surface with acetone and a clean rag to remove any wax, oil or grease
which might contaminate the repair. When this dries, chip out all the
loose material with a knife, then use 40-grit paper to bevel the edges to
a taper. The scratches should reach down into the crack or hole for better
bonding strength. Even thin cracks will have to be "opened up"
before material can be added to fill them. Blisters will have to be
completely exposed at this time. Also, be sure the remaining laminate is
solid and dry. Wipe down the surface one more time to remove dust and
anything else which may disrupt adhesion.
3) Mix and apply the appropriate cosmetic filler material.
Shallow scratches can be directly filled with colour matched get coat.
Be sure to match the gel coat to a sanded and polished portion of the
original, not the overall faded colour. The repair may stand out at first,
but everything will fade to the same colour in about a month. Use a small
brush to dab in the gel coat. Be sure it is higher than the surrounding
surface so it can be sanded flush. Spray a light coat of PVA over the
repair for a tack-free, sandable cure.
Deeper gouges will require structural putty filler to replace the
missing material before the gel coat is applied. 1/32 inch Rayplex milled
fibers in resin make an excellent structural putty. Use a squeegee to
spread it into the bottom of the gouge. Keep this filler slightly below
the surrounding surface if gel coat is the intended top coat. When
painting, the filler can be level with the surface because the paint adds
little thickness. When filling hull blisters, be sure to use Isophthalic
resin to make the milled glass putty. This will add extra corrosion
protection to these weakened areas.
If the cosmetic repair is following a structural repair, it may be
necessary to level the structural patch with the solid laminate. Both
grinding and filling may be necessary to accomplish this. Grind all high
spots until they are flush with the rest of the surface, unless this will
compromise the structural strength of the patch. Use filler to level any
low spots. If grinding cannot take place for structural reasons, fillers
can be mildly added to smooth irregularities, but the patch will always be
visible. On marine repairs, use the 1/32 inch milled glass putty filler
described previously. Other structures can usually be filled with standard
pre-made talc body filler unless otherwise specified. Continue adding
filler and sanding until the surface is perfectly flush.
Epoxy repairs are quite stable once they are cured, but polyester
repairs should be heated slightly before final finishing. The polyester
resin in the repair patch and in standard body filler continues to shrink
for some time when exposed to heat. When the repair is nearly finished,
sit the project out in the sun for a few days or set up a heat lamp. If
the lamp is used, don’t put it too close for too long. The concept is to
"force" the filler to shrink before it is sanded to the perfect
contour. Then, go back and sand the patch to the exact final shape. The
surface is then stabilized against future heat problems.
4) Mix and spray the colour matched top coat.
If gel coat is the intended top coat, repairs are best sprayed on using
an inexpensive automotive siphon-feed gun. This type of gun permits
adequate control of the process and can usually be purchased for less than
$50. It is necessary to thin the gel coat with Duratec Hi-Gloss
Additive to spray it through this type of gun. The additive will also
reduce orange-peel and later sanding to remove the texture.
Below-the-waterline repairs on marine projects need to use a surfacing
agent like PVA or wax to ensure a thorough cure. Wax is excellent on
large, hard to reach areas, but it can cloud clear gel coats. PVA is the
best for spot repairs where access is easy while the gel is still wet.
If the paint is the top coat, a surfacing primer is worth applying
underneath. The primer will hide the last fine irregularities and seal the
patch. When working on polyester parts, Duratec Surfacing Primer is
the best material for the task. It has a higher heat distortion
temperature, which further protects already stabilized patches from
shrinkage. Most paint systems will stick to cured epoxy, so apply the
primer recommended for the paint that will be used.
Blistered boat hulls should be covered in a layer of Isophthalic
resin for maximum protection against future water intrusion. Once the
barrier of Isophthalic resin is in place and sanded, it can be covered
with polyester gel coat, conventional anti-fouling bottom paint, or a
polyurethane automobile finish if it is not left in the water for extended
Gel coat will need to be finish sanded prior to final buffing. Use a
hard rubber sanding block and 400 grit wet/dry paper to level the gel
coat. Focus the sanding effort only on the high spots until everything is
flush. Then, switch to a foam sanding pad and even finer paper for the
perfect finish. Be sure to change water when the paper is changed so the
dust particles from the coarser paper do not continue to cut the surface.
5) Polish the repair area to the desired luster.
The last step of a professional cosmetic repair is buffing and
polishing. Our Meguiar’s MGM 1 & 3 are perfect for both polyester
and epoxy patches. Use only MGM 3 when polishing out a paint finish, but
gel coat needs both compounds. The fine scratched left by 600-1000 grit
paper will quickly disappear with the Step 1 Polish. It will even help
remove overspray and some of the halo left after performing spot repairs.
The finer compound gives it the final shine!
Get Out There And Fix It!