The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

104 Core Boatbuilding Techniques
If the parts to be joined are uncoated, it’s a good idea to
apply an initial coating of epoxy to the area of the joint.
When fillets are made on bare wood, the substrate may
absorb too much epoxy from the filleting mixture. <strong>The</strong>n,
the mixture thickens and becomes almost impossible to
work with. If the wood has been precoated, as described
in Chapter 15, you will need only to sand the joint area
to be sure of an adequate bond.
When the parts are ready, mix the epoxy and appropriate
fillers. Good, smooth fillets depend on the correct thickness
of the filleting mixture. With a little experience, you
will quickly learn what works best for you. <strong>The</strong> viscosity
of this mixture becomes more crucial with larger fillets.
As the mass and weight of your epoxy/filler combination
increase, so does its tendency to sag.
One solution to this problem is to make the mix even
thicker, but this results in fillets which are difficult to
apply and which will require a lot of clean up later. A
thick filleting mixture may not properly penetrate the
substrate and so may result in weak bonding. We find
that a two-step installation procedure, applying about
60% of the joint on the first pass and the remainder
when this stiffens up, is effective in this situation.
Begin making your fillet by applying a liberal amount
of fillet mixture to the mating edge of the joint. <strong>The</strong>n
position the part so that its surmarks line up exactly,
using staples, small nails, or clamps as appropriate to
hold it temporarily in place. Much of the epoxy/filler
mixture you use here will squeeze out and become part
Figure 11-19 A disposable syringe being used to apply a
bead of thickened epoxy.
of the fillet; the rest will fill any gaps between the two
parts. Next, apply fillet mixture all along the line of the
joint in the quantity needed for the desired fillet size.
Most often the mixing stick is used to move adhesive
from pot to joint. <strong>The</strong> WEST SYSTEM 804 Mixing Stick
has a round end for applying and shaping fillets and a
beveled square end for scraping up excess epoxy.
Continue using your stick to smooth out the fillet and
to remove excess material and redeposit it where
needed. Applying just the right amount of epoxy/filler
blend to a joint is one of the keys to filleting success. It
is better to add just a little more than to end up short
because it’s more difficult to add material later than to
remove a little excess.
In the past, we have recommended a variety of tools
for fillet application. Disposable cake decorating bags
are clean and accurate for laying down a bead of fillet
mixture, but they are difficult to find. Syringes, too,
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Joint coated with epoxy and
temporarily held with staples
Excess
Tangent point of fillet
stick produces a clean
margin area that
aids in the cleanup of
excess epoxy mixture
Fillet
stick
Sharpened tongue
depressor or putty
knife
Figure 11-20 <strong>The</strong> proper sequence for (1) coating and
joining the parts, (2) applying and shaping the fillet and (3)
cleaning away excess epoxy outside of the fillet.
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