: As far as the surfboard is concerned, I don't see the point of
: sealing the inside surface. Since it is a closed body, the point
: is to keep water from entering by sealing the outside. If it
: gets inside due to a failure of the outside surface seal, then
: sealing the inside will trap moisture within the wood, leading
: to rot. I think what is needed on the inside is a good anti-rot
: treatment as a backup, and a small hatch for drying the inside.

Should have mentioned that it is a hollow surfboard. This means that it will require venting. In theory it should be completely sealed but in practice areas like vent plugs/, leg leash points could be susceptible to a leak every once in a while. The trapped air is constantly heating/cooling/shrinking and contracting, especially in our sub tropical climate. I have just repaired a small leak on my surfski. Not the first time I've had water get into a "sealed" craft and probably won't be the last. I posted the same query on another site as well and got an interesting reply to do with the volatility and eventual failure of oil. Have advised my mate to seal internals with paint.
when its all said and done, my mate doesn't even surf. He and a couple of other will try it and I expect it will end up hanging above the bar in his pool room.
Cheers