I'm helping a couple of the blokes at work build surfboards. The latest board (6" fish) is being skinned with 3mm pine ply top and bottom rather than strips. Reason being, we are looking to make the process as quick, cheap and easy as possible for students to build. We will be applying 2 coats of epoxy followed by a varnish. Can't see any reason to glass.

Regarding the sanding, we use  ros with 100-120 grit to knock it down thend block sand with 240 lengthways. The boards come up a treat.

Thanks. I've read conflicting advice on this. Some say just apply varnish only while others say you need to apply epoxy to seal the wood to prevent water stains over time. Interlux says to apply their sealer product though that's an additional $35 for a can. Surprised how much more expensive maritime wood finishing supplies are compared to finishing supplies for wood furniture.  

We use an exterior grade varnish from Bunnings ( our version of Home Depot). I'll be using the same stuff on my latest kayak. Others may disagree, but the top end marine grade varnishes are overkill for craft that are generally babied and stored out of the waterand undercover.

I would personally go the epoxy +varnish rather thn varnish alone.