If I was going to do an entire boat, I would paint it. The finish work is simply easier and you don't have to worry so much about matching. In the case of repairs or doing only part of a boat that is already gelcoated, I would gelcoat.

As far as the gelcoat goes, I gelcoated the bottom and up about 1/2 of the hull sides on a Nacra 20 (blue). The gelcoat I used covered MUCH better than the other gelcoats I have previously worked with and it took about 1/2 gallon to do both hulls. I'm guessing that a gallon may be around 12 to 15 lbs (5.4 to 6.8kg)...I used about 1/2 of that weight on the parts of the boat I sprayed. We did a white boat at about the same time and it took an entire gallon to cover a little less area.

I figure I sanded away about 1/3 of that gel (perhaps?) so we're talking about 2kg or so added to cover the bottoms and taper up the sides of both hulls.