: Paul:
: You will need to eliminate the mold before you can do anything.
: Chlorine bleach will do it, but could also affect the color of
: the wood. Don't be afraid to paint it though. Some of the most
: stunning plywood kayaks out there are painted. I can tell you
: from experience that it is also a lot easier than varnishing.
: Use a high-build primer, sand most of it off, then roll on the
: paint and tip it out with a foam brush. -Wes

That method worked for me. I had several sheets of water-stained ply I got half price and accidently used a sheet for the long seat of my sailboat. Did not notice until glue had dried but there was a dark spot about a foot square right in the middle. I used a weak solution of bleach and dabbed it on sparingly with a moist cloth so it wouldn't run and mess up something else. I applied around a dozen treatments over a few days. The area looked slightly paler than the rest when it dried but that disappeared with the first coat of varnish so I bright-finished it. Now even I can't find the bad spot. Wear latex gloves and cover other areas to protect from drips.