The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 15 – Coating and Finishing 159
most paints and varnishes cure without a problem.
When using the other hardeners, paint behavior can be
unpredictable. We can’t test all of the many brands on
the market, so we urge you to experiment with your
planned finishing system before you make a major
investment in it.
Linear polyurethane (LP) paints are without question
vastly superior to any others we have used. We recommend
LP systems highly. <strong>The</strong>y have excellent resistance
to sunlight, salt spray, and other weathering agents.
Because they are as hard and resistant to abrasion as
polyester gelcoat, they are used almost exclusively for
recoating fiberglass hulls. Linear polyurethanes bond
extremely well to properly prepared WEST SYSTEM epoxycoated
surfaces and typically cure without a problem.
LP paints may be sprayed, rolled, and brushed. Hand
application is slower than spray painting, but requires
less preparation, so there is not a great time difference
between the two methods. In some situations, brushing
may be quicker. In all cases, it is far safer.
Hazardous solvent fumes escape from any type of paint
as it is applied, and care must be taken to provide
adequate ventilation during painting operations. Linear
polyurethanes present additional health hazards because
they contain 1% to 2% isocyanate, a highly toxic
material. When LP paints are sprayed, isocyanate is
atomized and may be inhaled. Most LP manufacturers
recommend that spray painters wear forced air respirators
while working, but we recommend that you simply
do not spray LP paints; roller and brush application of
linear polyurethanes substantially reduces the problem.
Clear Coatings and Natural Finishes
Clear coatings, including varnishes, are attractive,
durable, and fairly easy to apply. <strong>The</strong>ir major drawback
is that they break down under exposure to sunlight and
therefore require more maintenance than pigmented
coatings. We share the desire of wooden boatbuilders to
show off the beauty of the material, but because a major
goal of our philosophy is to keep things practical, we
prefer to use clear coatings in interior and other areas
where the effects of ultraviolet rays are minimal. See
Chapter 26 for a discussion of varnishing interiors.
Two things happen when a clear-coated surface is
subjected to too much sunlight. <strong>The</strong> first and most
obvious is that the varnish itself gradually degrades. At
the same time, in a less well understood process, slight
chemical and physical changes take place in the surface
wood cells through the varnish. <strong>The</strong>se changes may
contribute to coating failure by promoting the physical
separation of an already-weakened coating from the
ultraviolet-altered wood.
With the limitations of present technology, all clear
coatings on wood may be expected to break down in
the presence of sunlight. <strong>The</strong> time this takes depends
on the formulation of the specific coating, the number
of hours it is exposed to sunlight, the location of the
varnished item on the boat, and the surface under the
clear coating.
Some clear coatings have longer life spans than others.
Those which last the longest contain expensive ultraviolet
shields. Sold by most marine paint manufacturers,
these are usually called spar varnishes; one of the easiest
ways to recognize them is their high cost. Most spar
varnishes have a phenolic resin base mixed with tung
and linseed oil, a combination which is less hard and
durable than LP paint but which historically has the
best track record among clear coatings for withstanding
sunlight. <strong>The</strong>re are now a number of clear linear
polyurethane paint systems formulated with UV filters,
which provide even better UV resistance when used
over epoxy.
Varnish will last longer on hull topsides than on decks.
Decks receive direct exposure to sunlight, but hull sides
take only glancing blows and are usually partially
shaded. <strong>The</strong> early morning and late afternoon sun that
shines directly on topsides is not nearly so destructive
as midday light because its rays are well filtered by
atmospheric interference. For these reasons, two coats
of spar varnish over WEST SYSTEM epoxy on a deck
might last only a summer in our Michigan climate but
twice as long on the hull sides.
You can cut down on labor and extend the life of your
varnish considerably by adding a new coat every year.
At the beginning of every season, lightly sand your
clear-coated areas and apply a single coat of varnish.
Using WEST SYSTEM epoxy as a base for varnish appears
to lengthen the life of a clear coating. We feel that the
reason for this is the very tenacious bond that is made
between the epoxy and the wood fiber. It seems to be