I think there's a point that's missed being said explicitly here, although it was behind a comment already made. It's not just about weight savings, it's about being able to ensure a good enough bond between the wood skin and the foam core while using less epoxy. Without sealing the skin first most of the epoxy would tend to soak into the wood versus the foam so the bond isn't as good with the same amount of epoxy - meaning you'd have to use more epoxy, hence adding weight. By sealing the wood, you limit a bit how much the epoxy soaks into the wood (of course it needs to do that to establish a bond), so the epoxy can grab into the foam as well, and you can be surer of a good bond between foam and wood with less epoxy!

Hope this wasn't too confusing. Plus I may be wrong - anyone have another view about this?

Cheers,
Ken