I've done both both a 19' fiberglass hull and a some kind of poly-plastic canoe by mad river, with krylon spray. On the fiberglass, it worked well, i did it over gel coat in most places, and directly over the glass in others. It'll chip of in spots, especially around sharp edges and where my anchor chain bangs around. But i'll just touch it up now and then with whatever colours I have left.
On the canoe, it looks great, but isn't too durable. But I drag that thing everywhere.