: How long should the epoxy be allowed to cure before being
: clear-coated?

It depends on the brand. West Systems says only 2-3 days. I don't think anyone would recommend that, necessarily, but if you're in a big hurry.

: How much UV exposure can it handle if I go out and paddle it before
: adding the clear-coat UV protection?

I think it depends on the epoxy. I know plenty of people who have paddled their kayaks in bare epoxy for whatever reason. West said 105/207 should be safe for 4-5 weeks of normal use, but after that you'd just need to sand any oxidation before adding your finish. I don't know about other brands, so best to give your manufacturer a call.

: Is there any reason I can't use something like 303 protectant for
: UV protection, directly on the epoxy layer, for the rest of this
: season, then clean it off before clear-coating for a more
: permanent UV protection?

I doubt you'd ever get it off and I'd doubt if anything would stick to it. Why can't you just hurry on a coat or two of varnish for protection now, and then make it pretty with more coats and sanding during the off season?

m