Hi Bob- if you've been lightly sanding in between coats & wiping it down with a clean cloth, I'd continue coating with the unmixed spar urethane. The "smooth" finish part may take more than one more coat on the oak. It is a representation of how clean the wood is & how 'level' the coat of urethane is on top of it. This is where the "light" sanding comes in.

How much time have you had in between coats?

Brad

Wrench Fetcher, PhD