I have gotten excellent performance from a one-part marine polyurethane (Interlux, if I recall correctly) that was sprayed on. Had a bit of problem with sags in a few spots (user error - use thin coats!!), but I have 5 coats on top and 4 on the hull, three + years ago, and the boat still looks new (except where I've dinged/scratched it in the course of using it). My boat is garage kept when it is not in use.

 Oh, and I guess you've seen this:

http://www.clcboats.com/shoptips/boat-paint-varnish-finishing/painting_polyurethanes.html

I use Helmsman Urethane Spar Varnish from Minwax. It's one of the few varnishes I've tried that dries like it should, is easy to find locally, and can be sprayed without special thinners.

   Chris

if you want to try something very durable, and glass clear, with a super hard finish, try SM1000 Revolution from Supermarine Paints.

 http://supermarinepaint.com/clears-revolution-SM-1000C.asp 

Read the instruction! put on very thin. The stuff is a bit expensive, but the gloss finish, with the UV protection can't be beat. I won't go back to any other coating, myself.

Just my 2 cents.

Lance

Be advised that the Supermarine clear is not compatible with varnish.  I used it on my first boat and it was very difficult to get acceptable, smooth results and I am stuck now because unless I want to sand back to bare wood I cant varnish over it.  In my case the SM is less abbrasion resistant than varnish.  The UV protection seems fine.  Overall, from my experience with it I can not recommend it. 

You are correct, it is not compatible with varnish. You have to make a choice, varnish or SM1000. Personally, I do not like the amber tones that varnish puts on the wood. I wanted  clear, so I could see the true color of the wood.

As for the sanding, you need only get down to the epoxy, not the wood. Once down to the epoxy, the finish is gone, and your varnish will work like normal.