Gel-coating the outside of an entire hull is not a home workshop operation. It would have to be sprayed very evenly, run "hot" to harden quickly without peeling, and mixed with surfacing wax or the outside would never completely harden. The finish will be orange-peel textured after it hardens and it will all then need to be sanded flat and buffed back up to a gloss finish. It's a major job and requires a fair amount of skill or it will be a mess. There are often two to three pounds of pigment in the gel coat layer on a typical fiberglass canoe (varies by color).
On the other hand, paint can be applied with no more than a thin foam roller and a soft brush to tip it out (or even just with a brush if you're really good at brushing) and after two or three coats it will look great. It won't be quite as hard as gel coat, but the difference in the application process, the potential for it to look good, and the relative ease of eventual repair is drastically better. It also adds less weight, because it is thinner than gel coat needs to be.
Unless you have a lot of small surface imperfections to fill and sand smooth (which you should not if you did a good job of sanding) there is no real benefit to using primer between a good paint and the fiberglass. It doesn't make paint stick any better to the glass than it will by itself, and it doesn't make the paint tougher. It just adds extra work and cost to the project.
For paint, you basically have three choices. The toughest is a two-part linear polyurethane paint, such as those sold by the marine paint companies. They are the hardest, but most expensive. They are quite toxic to spray, but will go on nicely rolling and tipping with a much reduced health hazard. Even so, they may be more complex to apply then you want to get into for a limited amount of extra durability.
Option #2 would be a one-part marine enamel. This can be a traditional enamel like Kirby's, which a lot of folks use on wood/canvas canoes, or a more modern marine one-part urethane (Brightside and Easypoxy being the most popular). They will go on very nicely. Brightside is my favorite on fiberglass surfaces, rolled and tipped-out, though their color selection is somewhat limited and I often end up mixing my own blends. These marine enamels are getting expensive these days, but really do a nice job. The dark green on the sailboat hull is Brightside, applied out in my driveway. When this photo was taken, the paint job was already three years old and had been sitting outside year round, as well as out on a mooring the previous summer. It was in for spring clean-up and still looked quite good.
Option #3 would be to use a polyurethane enamel (like floor enamel) from a hardware or big box store. These may not be quite as smooth or quite as glossy as the marine enamels, but they are reasonably close, decently tough for paint, and a lot cheaper. You can buy a gallon for the price of a quart of marine enamel - and they can mix you just about any color you can dream up. The yellow Old Town is Ace Hardware polyurethane floor enamel over sanded WEST epoxy/fiberglass, rolled and tipped with no primer used. It dried with a bit more orange-peel texture than Brightside has, and a bit more of a satin look, but still looks good. Different brands seem to vary a bit regarding finished texture and gloss, but they're all reasonably durable as long as you aren't running your wooden canoe through rock gardens. Pretty easy to touch up as well. The "birchbark" stripper canoe has a base coat of Home Depot "Epoxy one-part Concrete and Garage Floor Paint" over the fiberglass, followed by a little bit of spraying in spots for color variation, followed by rolling on the little graining lines, followed by a light sprayed coat of satin conversion varnish to eliminate the gloss.
Even though any paint will scratch, I think you'll probably find that some sort of enamel is going to be your best bet. Gel coat is a pain in the rear to work with and isn't all that much better at avoiding scratches or adding durability when applied after the fact to an old hull. Plus, repainting when needed is pretty easy. My Old Town has worn three different color schemes over the last ten years. If I get bored with one, I pop the outwales off, spend about three hours with the random orbit sander taking it back down to bare fiberglass and can re-paint it the next day.
