While I'm lucky enough to be able to store my boats under cover, the strip-built canopy on my pickup truck has spent considerable time exposed to the sun and weather for over two years with no detectable deterioration. I used Epiphanes varnish, but I would expect similar results with Captains.

Having said that, I would recommend at least a loose-fitting tarp that would still allow ventilation underneath it, so the boat could dry out if it gets damp. Also, if your un-covered boat is only partially exposed to lots of direct sunlight, you might get a "shadow" effect over a long period of time. The tarp should prevent that.

Of course, if you need a good excuse to build a boat storage shop, er, I mean, garage, then this might be the one!

Pete, in Snohomish