I have some preliminary test results of various coatings for polyester skin. I did 4 samples, water-based urethane (Varathane Outdoor Diamond Wood Finish), oil-based urethane (Minwax Helmsman Spar Indoor/Outdoor), spar varnish (Z-Spar Captain's), and elastomeric roof paint (Snow-Roof, primed with Elast-o-Seal).

Application:
Water-based urethane - This was easiest, I could reapply in as little as 2 hours or as many as 24 without sanding. Requires 5-6 coats to get a smooth finish.
Oil-based urethane - Reapply between 4 to 12 hours if you don't want to sand. Requires 4-5 coats to get a smooth finish.
Varnish - Reapply every 12 hours to avoid sanding. Requires 4-5 coats to get a smooth finish.
Elastomeric roof paint - Very thick, filled the weave quickly, but couldn't recoat quickly if put on too thick. Requires 4-5 coats to get good coverage. leaves brush strokes or weave showing.

Appearance
Water-based urethane - Feels plasticky, slick, somewhat stiff, but flexible without cracking. Clear.
Oil-based urethane - Feels stiff, but didn't seem to want to crack, it may with age, this takes longer to cure. Yellow.
Varnish - Very similar to oil based urethane. Amber
Elastomeric roof paint - soft, rubbery, heavy, and limp. May have stretched the polyester when applied. Pimer Grey-black, topcoat white.

Punture test *
Water-based urethane - hardest to punch a screwdriver through. Separated from polyester at puncture. Did not want to continue ripping.
Oil-based urethane - Almost as tough as water-based.
Varnish - Almost as tough as oil-based urethane. Could continue ripping
Elastomeric roof paint - Easiest to puncture. Did not want to continue ripping.

* This was only one day after final coat was applied. Some of these require more time for a complete cure.

I am leaning towards the water-based urethane for this kayak. Roof paint is cheaper, but water-based urethane looks like it is better on polyester.

Mike