The super thin stuff would sink into the wood more, and harded it up more, rether then create a hardened layer sitting ontop the wood. I personally would prefer the fingerboard wooditself to be hardened up, not just have a layer the could eventually flake or wear off.
Yew, I would use a saturation system epoxy. Also, the fret board is rediused now, so I would just sand the epoxy to do a final radius.