FC,
I own a plywood boat so I have to protect a lot of wood (cabins, decks, hull etc
).
What works for me is:
Coat bare plywood with epoxy (at least two coats, wet on tacky). Make sure you are using an epoxy formulated for wood, it then should have similar co-efficents of expansion (that is for "average" wood, as woods do vary). By the way, epoxy bonds better to freshly coarse sanded timber than unsanded or old sanded or fine sanded or even freshly planned wood.
If at all possible, coat ALL sides of the wood -inside, outside and all edges. Sometimes you can't achieve this but do the best you can. This really helps to stabilize the wood so the only expansion / contraction will be due to temperature and not moisture. Fully epoxy coated wood will maintain the moisture content of the day it was coated.
Then after curing, remove epoxy "blush" if you epoxy creates this. Use fresh water and nylon scourer. Better to use a "blush" free epoxy but there are not many of them around despite the manufacturer's claims.
Now lightly sand the epoxy to remove gloss surface and give the primer something to key to. NEVER try to sand the "blush" off, it is a wax and you just distribute it right into the sanding groves and it clogs the paper as well.
Then two coats of primer - I use wood primer suitable for below the waterline but many people use a two pack epoxy primer (which I believe is better but I just prefer single pack paints).
Now top coat with your choice of paint. Again I use two coats of a single pack alkyd undercoat/surfacer followed by two coats of a single pack epoxy modified alkyd gloss coat.
This method has lasted me 7 years so far with only an light sand and another two top coats every 12 to 18 months.
Others have different ways but mine works for me
.