J. E. Moser makes a water-based spar varnish I've used on an Alaskan Recovery kayak frame... easy to spray, quick build-up of coats, and slightly ambered to look like a traditional spar. I also used it on an exterior door to our house as a more rigourous test... so far, so good. No signs of chipping or peeling after 12+ months. Of course, convenience comes at a price: $63 a gallon through Woodworker's Supply. It's called MarineShield.