I once had a conversation with a person from Wind Song Canoes.

http://www.woodsongcanoes.com/wood_song_contents.htm

The boats looked like they were dipped in a vat of plastic. Fantastic workmanship and unbelievable varnish jogs. The owner told me that they apply varnish out in the sun with a spray gun and coat after coat with no sanding until they get a thick build up. Bugs in it whatever. Then they sand that and give it the one last coat inside.

BTW: my favorite so far is Epiphanes. Polyurethanes are synthetic and harder than regular varnish. It was invented for floors. Varnish is supposedly made from natural ingredients like pine tree stuff etc. Regular varnish is more flexible and better on boats where the finish is applied right on bare wood where there is shrinkage and expansion. Hence the name "Spar Varnish". Polys are fine on glass or epoxy coated surfaces but either is fine for what we build.