The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 15 – Coating and Finishing 157
place, amine blush may clog sandpaper, and it may also
jeopardize adhesion. This residue may attract moisture,
which can also prevent proper adhesion of subsequent
coats of epoxy. Amine blush is most evident when a
coating cures in cooling temperatures, so maintaining
constant environmental controls as epoxy cures aids in
preventing its development.
In other situations, surfaces may be contaminated by
substances that are not water-soluble or by water itself.
To remove non-soluble materials, use an appropriate
cleaning solvent with white paper towel. In high humidity,
moisture may collect on a surface. This will act as a
contaminant and interfere with bonding between coatings.
Condensation may cause clouding in an epoxy coating.
When moisture is a problem, use 205 Fast Hardener
since this cures more quickly and has less “green” time
in which to collect moisture or use 207 Special Coating
Hardener, which is less affected by moisture. Try also
to maintain more constant temperatures. If for some
reason you are working outdoors in the rain, cover
freshly coated areas with a tent of plastic sheeting.
To eliminate any potential contamination problems
between coatings, it’s easiest to recoat while the
previous coat of epoxy is still “green.” This can be
anywhere from 1 to 3 hours after the earlier application,
depending on temperature and hardener. When the
epoxy begins to cure and becomes tacky, you can
immediately roll on another coat without having to
worry about running. Several coats of epoxy are possible
in a single day with only short intervals between them,
with no sagging. We have found that if you roll on the
epoxy and tip it with a brush or segment of foam roller
cover, air bubbles and roller stipple are eliminated.
<strong>The</strong>n the self-leveling ability of successive coatings is
sometimes as good as, if not better than, a single coating.
When you have applied the proper amount of epoxy,
your next step is to smooth the surface for painting or
varnishing. In many cases, especially in the interior,
you will have sanded most of the parts before installing
them in your hull. It is usually most efficient to sand
exterior surfaces on the completed hull after all coatings
have been applied.
General sanding of a cured epoxy surface can be a lot
of hard work, especially if you are working towards a
high-gloss final finish. <strong>The</strong>re are, fortunately, some
laborsaving shortcuts. We use a foam disc grinding pad
Figure 15-5 Finish sanding gray pigmented epoxy on the
deck of Slingshot. Orbital finish sanders are very effective for
such finish sanding.
on a light-duty power circular polisher to rough and
smooth-sand 90% of the hull’s exterior surface areas.
Usually, we use 80 to 100-grit sandpaper with the
sanding pad to first rough sand coated surfaces, and
then use 120 to 180-grit for power finish sanding.
Many boatbuilders find this surface adequate. A further
wet sanding by hand with 220 to 320-grit paper will
produce an extremely smooth surface for a high-gloss
finish for either varnish or paint.
A good way to verify that the surface is contaminant
free before painting or varnishing is to wet it down with
clean water (no soap). If it is contaminant free, the water
will sheet off and be perfectly smooth. If it is contaminated,
fisheyes will appear or areas will bead up rather
than laying out in a smooth sheen. If contamination is
present, wet sand or scrub the area with an abrasive
pad and water until the surface is contaminant free.
Primers
We find that we rarely need primer paints when we
coat hulls with WEST SYSTEM epoxy. Primers can’t, of
course, be used on naturally finished hulls, but even on
painted surfaces, they don’t seem to result in any particular
saving in finishing time.
Primers are not usually needed to help bond a paint
film to cured WEST SYSTEM epoxy. Most finish coatings
readily adhere to a clean and well-sanded epoxy-coated
surface, although interfacing primers are required with
some specialized bottom paints. If the instructions on
your paint or varnish recommend a specially primed
surface, sand your epoxy coating until it is no longer