While Zar is probably the most popular of the 'hardware store' polyurethanes, its not the only one that will work. Zar can be difficult to find, but calling around to your local hardware and paint stores might turn up a retailer that's not on the Zar website. The interior versions will work on a skin boat, just don't leave it outside in direct sunlight for long periods of time...

Another alternative is Rustoleum Marine Spar Varnish. Its not the regular Rustoleum line, but a "marine" line of boating-specific coatings. Its sold at some big box hardware stores (I find it at the "blue" one) in locations near large lakes or coastal areas. My own testing reveals that it stays very flexible over time (6 to 7 months so far) and does not crack or peel even when folded over and creased. I'll be using it on my current build, over linen fabric. The regular Rustoleum products have mixed results on skin boats, for what its worth.

As for sanding, the product you end up using will dictate the application process. Most (but probably not all) can be recoated without sanding within a specified time; just read the directions to make sure.

: I am about ready to coat my kayak, and am trying to find a good product that
: I can pick up locally. I've been searching the archives and see that Zar
: seems to be highly popular. However, I searched the manufacturer's site
: and it looks like there are no suppliers in my area.

: What are some other good oil based exterior polyurethanes? I am trying to
: finish up the boat in the next week so I can take it with me when I travel
: the following week.

: Also, if I don't let it cure till its totally hard (for better cohesion with
: the next coat), do I need to sand between coats?

: Thanks!