I have no experience spraying varnish, but I do know that the pros brush it on.

I sprayed my first boat, but have used a brush on my last three. If you create an enclosure to hold down the dust, roll the finish on with thin coats, and tip out with a foam brush, you can actually get a better finish than spraying most of the time. Best of all, you avoid the messy cleanup. Interlux Enamel and Minwax Helmsman Urethane both level out very nicely if you lay them on quickly and leave them alone. -Wes

You can get an especially beautiful finish by spraying either varnish or a good automotive acrylic urethane over the fiberglass finish. My 19 ft kayak is 21 years old now and the finish still looks like new. I used a product called "Diamond Hard" which cured to a very hard finish and has been very durable. The resulting finish is mostly "bright works" wood and colored bottom.

I sprayed the hull first, masking off the deck. After the first coat of clear, I masked off a nice water line and then sprayed the bottom black. I then applied vinyl masks for the name (you might want some other graphics too) and then sprayed them white. I removed the maskes while the color was wet, but not sticky, and once set, resprayed the hull clear again.

Once dried, I back taped and masked off the hull and sprayed the deck as I did the hull making the demarkation line underneath the later applied rub rails.

Because you are painting over fiberglass and not wood, an automotive clear coat works much better than traditional varnish. In the past I have used epoxy varnish, brushed, sanded and then the last coats sprayed over wood and it yields a similarly beautiful finish, although not as durable as Urethane.