the wet-out coat to partially cure. When it
has stiffened but remains tacky, the blister is
ready for filling.
Thickened epoxy resin alone is suitable
to replace the gelcoat and the first layer or
two of the underlying laminate, but where a
blister involves deeper layers, it’s good practice to replace the fiberglass reinforcement
that you ground away. It’s not necessary or
even particularly desirable to try to match the
original lay-up schedule when repairing blisters. Binders in fiberglass mat often make it
incompatible with epoxy, so the best choice
is six- to 10-ounce fiberglass cloth. Cut
the cloth into discs slightly larger than the
flat part of the depressions and press them
into the tacky wet-out coat. Saturate the
cloth with epoxy resin, thickening the resin
slightly with colloidal silica if you experience
difficulty with the resin draining out of the
weave. Add and saturate as many layers of
cloth as necessary to restore the laminate to
its original thickness (FigUre 5). Allow this
lay-up to cure for around 30 minutes before
filling the remainder of the cavity with silica-thickened epoxy.
For this you want to again mix up a small
The Final hUrdle
amount of epoxy resin, and then stir in col-
loidal silica (West 406, Aerosil/Cabosil, for
instance) to thicken the resin to the consis-
tency of creamy peanut butter. The filler in
this case is also the barrier coat, and colloidal
silica will not compromise the resin’s resis-
tance to moisture penetration. Use a flexible
plastic spreader to trowel the filler into the
cavity and smooth and fair it (FigUre 6).
Learn to curve the spreader with thumb
pressure to copy any curvature of the hull,
and take extra time to try to match the
surface of the fill perfectly to the hull. As col-
loidal silica makes the cured filler resist sand-
ing, extra time here pays generous dividends.
Epoxy does not shrink, so there’s no need to
overfill. A topcoat of unthickened resin after
partial cure of the filler can improve the fair-
ness of the repair.
That’s it! Let the epoxy cure overnight, then
use a Scotch-Brite pad and plenty of water
to remove the waxy amine coating. Follow
that with a bit of block sanding with 100-
grit paper to fair and scuff the surface of the
epoxy, and you’re ready for bottom paint.
Fixing existing blisters is the first step
toward figuring out whether a more exten-
sive and vastly more expensive repair will be
required. At the very least, repairing individu-
al blisters puts off a more costly response for
another year. At best, it avoids that expense
altogether. Repairing a few new blisters every
year can be a low-cost, long-term strategy.
Sailor, author, and DIY guru Don Casey has
been a longtime contributor to BoatU.S. and
Ask the Experts.
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