Hi kevinonputnam, 

all the wood, wheter its glassed or not,  gets coated with epoxy prior to painting.   the epoxy seals the wood from water.

for this kind of wood construction (stitch and glue with marine plywood), the wood needs to be coated to keep it from deteriorating and to preserve its beautiful look..  even though it is a marine-grade plywood, it is not meant to be imersed without being epoxy coated.

hope that helps. 

h

   Yes, fully encase the wood in epoxy inside and outside. Preferably epoxy and glass to provide the longest life to your boat. That makes it a "composite" boat.  

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No, you don't have to epoxy the boat before you paint it. They made wood boats long time before there was epoxy and covdered them with paint....over....and over .....and over again. They, we, also replaced rot over and over.  

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You will find that when you epoxy and glass then paint most scratches are in the paint and/or varnish not the epoxy.  Your boat will last longer. It will aslo be stronger.  That and you won't get the leaks that make those black spots in the varnish. 

while they clearly made wood boats before epoxy....and painted them.... most of the construction was with timbers and boards and not plywood.   those whole wood products, if the right species and combined with the right techniques, had incredible service lifes if properly maintained (hence the whole wooden boat thing).

but i am not sure that 'marine grade' plywood would actually perform if submerged for any length of time compared to the whole wood product of older wood boats.

so i was not concerned about 'rot' as much as would the plywood begin to decay/have its layers unravel after extended submersion.

i just don't think of our plywood construction as a wooden boat.  its a sophisticated/modern construction technique that uses modern plywood and can capture that wood boat feel.

i am curious if anybody has insight on what would happen if you simply painted marine plywood....how long would it last or would it come apart pretty quick?

h

   Timbers?.......well I never got that serious.

For example, my  Y-Flyer, Y2375,  was a 18ft plywood  scow sailboat built in 1975 in South Carolina and christened in December of that year. It was a cold sail. The skin was 3/8" marine and exterior plywood. Exterior plywood has exterior waterproof glue, footballs and the possibility of voids. It was painted with a two part poly paint.  I used 2x treated pine for the CB trunk and tended to fiberglass some connections. It was heavier than it should have been. The pine transom often rotted out and required replacement. The stem often required attention to paint. 

My farther-in-laws Y - flyer, Y 860  was built in 1962 I think.  It was 3/8" marine ply painted with two part poly paint.  His boat was professsionally built before a lot of fiberglass was available.  It was a good hull and light for a wood Y-Flier.  The center board trunk connection to the skin had rot problems. Various areas of skin tended to need attention.  The spruce mast was heavy and varnished annually. It often had problems around the fasteners, bolts, screws, etc.  

Both boats were tender when around docks and would get scrapped and dented. Both boats required annual touching up of paint or repainting prior to the season. Both boatts were trailered. The old one had more problems around the bailers with soft wood and water intrusion.  

Now lets look at these composite boats.  My kayaks that have plywood are 4mm thick, less than 1/8 inch. (or 1/4" strips) . They are epoxy fiberglass encased. They get scratched but it is usually in the varnish and rarely in the glass.  The Jimmy Smith II is a little heavier ply than the kayaks and epoxy/fiberglass encased, everywhere. It is combination of painted and varnished .   It weighs in around 150#, or 11.5# per lineal foot.  It has a usuful load around 500#. The Y-flyer was longer, but had the same load rating and weighed 500#, 28# /LF. The JS often lives uncovered in the Florida sun in cradles at the bulkhead. It is 2 years old, used off and on all year and has not needed any work.  The  Y-Flyer lived in a garage went out in the summer and needed work every year. It had lots of ribs.  I remember having it in work when my friends came back from overseas for a short visit and we couldn't go out. 

The reason the skin is lighter is that the wood lighter and  is encased in epoxy and glass.  The epoxy makes it stronger and more water proof. I tend to glass more that specified.  The OP needs to encase the wood.  

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PS.......Y-Flyers transitioned to fiberglass hulls in the 70s. We were just hold outs.