The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

126 Core Boatbuilding Techniques
Figure 13-5 Roll up a half width piece of cloth on a section
of PVC pipe to make overhead application easier.
cured, 2 or 3 hours after application with fast hardener.
Excess cloth is much more difficult to cut when the
epoxy is fully cured, and it can be hard to peel it from
a surface where it is not wanted. Use a sharp blade to
cut away overhangs of epoxy-soaked cloth, keeping
the blade clean with solvent.
We usually lay cloth on one half of a hull in one operation,
overlapping it a couple of inches past the keel.
An irregular overlap can cause fairing problems, so
we trim the edge of the fabric parallel to the keel before
the epoxy has cured. To do this, we hold a light batten
parallel to the keel, cut along it with a sharp knife, and
peel away the excess. <strong>The</strong>n, when we apply cloth to the
other side, we lap it 2" (50mm) past the centerline for a
total overlap of about 4" (100mm), and even it off. Trim
the edge with scissors or lay a protective strip of wood
or cardboard under the edge if using a knife. <strong>The</strong>
extra layers of cloth help to protect the keel, which
takes a good deal of abuse.
Running out of cloth and starting a new roll halfway
down the hull can cause some problems. An overlap
will create a bump, which will require filling and
sanding, but it can be hard to butt two pieces of cloth
against each other. We’ve found that the best solution
is to fit the two pieces carefully, with as little overlap as
possible, and squeegee them in place. <strong>The</strong>n, when the
epoxy is partially cured, we slice down the middle of
the overlap and, working on each side in turn, lift the
fabric from the hull, pull away the excess and replace
the fitted portion. A little more epoxy may be needed
to re-wet the surface.
You can recoat the cloth with foam rollers when the
epoxy is tacky. <strong>The</strong> first roller coating will probably not
fill the weave of the fabric completely. Allow it to cure,
then wash and sand the surface lightly with a power
sander and soft foam disc pad, taking care not to oversand
any areas. Don’t try to achieve a totally smooth
surface—just remove the high spots so that the next
coating will flow out more evenly and less sanding will
be needed later. We usually apply two more coats of
epoxy in quick succession before final sanding, but
one is adequate in some cases. Coat the cloth heavily
enough to make sure that final sanding will not expose
any bare fiber and reduce the moisture vapor barrier
provided by WEST SYSTEM epoxy. <strong>The</strong>n coat and finish
the hull as described in Chapter 15.
Estimating Epoxy Amounts
<strong>The</strong> formula will help you estimate the amount of
mixed epoxy needed to wet out fiberglass cloth
(assuming a resin-to-fiber ration of 50:50) and apply
three rolled epoxy coats to fill the weave of the cloth,
i.e. “fill coats.”
<strong>The</strong> formula includes a waste factor of approximately
15%; however, more (or less) may be needed
depending on the job and personal application
technique. <strong>The</strong> epoxy is applied at standard room
temperature, approximately 72° F.
Gallons of mixed epoxy =
A 3 [(Wf 3 0.00085) + 0.0075]
Where
A = Total area covered by fiberglass. Units are in square
feet (ft2 )
Wf = Total weight (W) per square yard of fiberglass ( f)
cloth used in laminate. Units are in ones per
square yard (oz/yd2 ), i.e. 6 oz fiberglass cloth
weighs 6 oz/yd2 .
Note: a Group Size B resin and hardener makes 1.2 or
1.3 gal of mixed epoxy depending on hardener.