The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

288 Hull Construction Methods
Another method is to draw the stringer locations on the
interior of the ply, preferably after prefinishing it. <strong>The</strong>n
transfer the marks to the outside by measuring from a
point on the panel edge to a marked position or by
drilling very small holes in the stringers and connecting
them with lines on the panel exterior. Whatever marking
technique you use, be accurate. Although missed
staples can be removed and repaired, this takes time.
Many missed staples can create substantially more work
than the little effort needed to properly mark stringer
and frame locations.
Applying Panels
Use a high-density adhesive mixture slightly thicker
than heavy syrup to bond the panels to the framework.
If you have larger gaps to fill, you may need adhesive
with a thicker, peanut butter consistency. Wash off
previously sanded, epoxy-coated surfaces to remove
dust and dirt and apply thickened epoxy to all joints. If
the panels have not yet been coated, roll unthickened
epoxy on any bare bonding surfaces before applying the
thickened mixture. Use enough thickened adhesive so
that some squeezes out of the joint. This indicates a
100% bond.
It usually takes at least two people to maneuver a panel.
Try to avoid dragging it across the framework and
smearing the epoxy. If you have tacked the panel for
fitting and have left the nails in place, line them up
with their holes and insert them. If you do not have the
nails to guide you, line up surmarks and insert one or
two locating staples at various central points on the
panel to hold it in position while you check that everything
fits as it should.
With sheet plywood, we prefer to use narrow-crown
alloy staples, which can be left in place. Narrow-crown
staples cause less damage to the plywood and, if they
are slightly recessed, their holes are easier to fill and fair
over for a smooth, easy-to-finish surface. Staples left in
chines may cause problems if you round the joints, so
use wide-crown staples in this situation and remove
them before you begin shaping.
Staples that are three times as long as the plywood
thickness are usually adequate to hold the ply as it
bends over moderate curves. In areas where the curve
is so sharp that staples cannot hold the panel, use
screws and large washers or other suitable types of pads
for temporary clamping pressure and to distribute load
until the adhesive cures. You can use nails with plywood
pads, but we are not keen on the idea of using alloy
boat nails and leaving them permanently in the hull.
<strong>The</strong> main difficulty with boat nails is that their heads
tend to show in the joint and greatly detract from a
good surface finish.
After you have successfully bonded one panel to the
framework, immediately clean up the excess adhesive
that has squeezed out from between the joints. (See
Chapter 22 for further discussion of cleanup procedures.)
Installation of the second panel becomes more complicated
because of the added difficulty of performing the
bonding operation on at least one scarf joint at the same
time that you are installing the panel. You should have
already prefit and cut the scarf joint on this second panel.
(Remember, with prefinished panels, you must cut the
scarf joint on the opposite surface of the plywood on
the second panel in order for the scarf bevels to mate.)
With the second panel, it is worthwhile to make a dry
run to check the surmarks and make sure that scarf joint
alignment is still accurate. Note any differences and
change the surmarks to ensure an accurate final installation.
Be very critical of your scarf joint alignment to
prevent misalignment of the beveled scarf cut on either
panel, which would cause unfairness in the skin. Because
scarfing in place on the hull is a little more difficult
than in a more controlled situation, we always provide a
safety margin by applying an excess amount of adhesive
to both beveled surfaces of the panels to be joined.
Pay special attention as you apply pressure to the scarf.
Staple through a plastic-covered strip of plywood
slightly wider than the joint for more uniform pressure
over its surface. Use staples, nails, or screws to apply
maximum pressure through the joint to a solid member
under it. To bring the exterior edges tightly together,
use shorter staples which do not penetrate through the
plywood. <strong>The</strong> interior edges of the scarf will probably
need relatively little pressure because the curvature of
the hull usually squeezes them together. If they do not
fit tightly, remove excess adhesive and bring them
together with a plywood strip and shorter staples as
you did on the exterior. (See Figure 24-6.)