Although I can't scientifically prove it, I suspect the tarp option is not a good idea. I keep a few projects outside in the Boston area and I am religious about removing the snow load - this can be a pain when you are up to your waist in the latest blizzard. I have also had the filler on my prospector crack on the sunny side of the canoe only. I think it is the result of the quick extremes in temperature in the sun during the winter. Below zero at night to maybe 60F or so under the tarp a few hours later - you get the idea. I have also seen paint lift off a canoe under a tarp, maybe too much moisture collecting under the canoe and lifting the paint off. Your mileage may vary.
Fitz
