The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 12 – Scarfing 111
Production bonding needs to be well organized and
requires the cooperation of at least two people. First,
pre-sort the stock to be joined and lay out the individual
pieces on a flat surface opposite one another with each
of the beveled surfaces facing up and pointing toward
its mate. No doubt you will want to make up varying
lengths of scarfed material, and you should do your
planning at this stage to minimize waste when you are
actually assembling the scarfed stock on the boat.
When you have laid out and organized all of the stock,
establish a separate assembly area close by on a flat
surface and prepare the clamps and a staple gun which
shoots 3�8" (9mm) wide-crown staples. On the floor, lay
out wax paper or a polyethylene sheet that is somewhat
wider than the scarf joints themselves to prevent
inadvertent bonding. Make up the adhesive mixture
and apply it to all the upward-facing beveled surfaces,
following the application procedure previously described.
Move two matched pieces of stock to the prepared
assembly area and align their beveled bonding surfaces,
using the procedure previously described. After you
have inserted the locating staples, turn the stock up on
edge with the side of the scarf joint resting on the wax
paper or polyethylene sheet. Before you move another
Figure 12-5 Plastic sheeting woven back and forth between
stringers contains the excess epoxy runout and prevents
bonding of the stringers to each other. When placing this plastic
sheet, avoid pinching the plastic in any of the scarf joints.
pair of matched pieces into the assembly area, wrap a
roll of wax paper over the standing surface of the scarf
you have just lined up; this will create a barrier between
it and the next scarf joint when they are placed side
by side. <strong>The</strong>n lift this roll of paper up the side of the
second scarf joint so that you can nestle the third scarf
joint beside the second without any bonding occurring
between them. Continue weaving the roll of wax paper
back and forth between all the ensuing scarf joints you
line up.
When you have lined up all of the joints in this manner
and they are sitting on edge, position heavy pieces of
stock about twice the scarf length on each side of the
stack. <strong>The</strong>se serve as clamping pads to distribute
pressure evenly. Position clamps at either end of the
pads to keep the joints from sliding apart when
pressure is applied to the scarf area. Use enough clamps
to apply sufficient pressure over the scarf joint area to
ensure good bonds. When clamping a large number of
joints at one time, you may need to tighten the clamps
at several intervals as excess adhesive slowly squeezes
out of the joints.
Fairing up scarf joints
A scarf joint is almost finished when its adhesive has
cured. <strong>The</strong> final steps in scarfing are always fairing
and cleanup. Excess epoxy inevitably squeezes out
around the joint and must be removed. If you have any
misalignment within your scarf, you will need to fair
into the overall dimensions of the stock. We suggest
that you take care of these matters when the epoxy is
partially cured—5 to 8 hours after application the
joint should be strong enough to allow you to remove
clamps—because at this stage the epoxy is easiest to
sand and plane. After 5 to 7 days, the work will be
much more difficult.
Remove the temporary locating staples before you begin
fairing. Hand-held block planes are the basic tools
for fairing scarf joints. Using two of them, each at a
different blade setting, speeds up the fairing process
when you have a lot of scarf joints to do. Set the blade
on the second block plane for a finer cut to remove the
last bits of material. <strong>The</strong>n use an 11" (275mm) sanding
block with 60-grit paper to smooth out any plane
marks and finish the joint.