I have to agree with Mike and add that my first yak, the one I kept and paddle, the one I learned all about what not to do, is over 5 years old. We've gone some really serious miles together. After all these years...
Sanding varnish off is a pain in the ass! These days I just varnish the deck down to just below the sheer, the hull gets zero varnish. After every long trip I sand down the hull and rub on a layer of epoxy. The yak is either in the water, temporarily on the truck, or in the garage. Nothing's happened to the epoxy yet aside from gouges on rocks.

In other words: I'm lazy

: At a minimum, I plan to varnish the exterior plus the cockpit area. Is it
: good to also varnish areas that get less exposure? I suppose opened
: hatches would let UV degrade unvarnished storage areas. But how much
: exposure is the threshold for damage? And what about the underside of the
: deck?