Why not use epoxy? Epoxy sticks far better to cured polyester resin or gel coat than more polyester resin or gel coat ever will. Epoxy is more controllable, easier to modify with fillers for specific tasks, easier to apply and much less prone to shrinkage. You can certainly fix damaged polyester gelcoat with more polyester gelcoat, but to do it well or cover anything other than a small area, you need a lot of expensive equipment that most people don't have sitting around in their garages unless they happen to be in the fiberglass boat business. You can re-skin a fiberglass hull using epoxy, wind-up with a better, more durable result and the only tools needed are a roller and a $50 random orbit sander and a can of good marine enamel to top it off.
Gelcoat is a mixture of polyester resin, various pigments and cheap filler (usually talc). It has so much filler in it that it's not even particularly sticky (run into a dock with the stem of a fiberglass canoe sometime if you want to see how poorly it really sticks to the laminate underneath). It also isn't very flexible (again because of the high percentage of filler). The thicker it is, the deeper the gloss and shine on the hull, but the weaker and more brittle the gel coat layer is. The sun doesn't generally cause spider cracks. It's the unstable nature of the polyester/fiberglass hull under the gelcoat and it's tendency to shrink slightly over time that cracks the gel coat. All that filler won't allow the gel coat to shrink as much as the hull and the gel coat fractures. You can also get spider cracks from impacts. Again, it's the inability of the gelcoat to flex with the rest of the hull that causes it to fracture around the impact area and hulls with nice, thick, shiny layers of gel coat are the most likely to crack.
A coat of wax might remove some oxidation and make your old hull look shiny for a while, but it's not going to prevent spider cracks or have any long-term benefits for hull durability other than provide a very temporary and minor boost in UV resistance, at least until the wax has deteriorated. Luckily, these cracks are seldom of any structural concern. The gel coat isn't adding any strength to the hull and the laminate under it is already plenty sufficient to keep water out. You can cover an entire hull with spider cracks and it still won't leak.
The Fix - Small areas using matching polyester gel coat - sand down deeply into the cracked area. You may not need to sand all the way down to the bottom of the cracks or down to cloth, but you need to go deep enough that they are much smaller. Gelcoat can be brushed on or sprayed with a small "Prevail" canned spray system (marine suppliers). After curing, you wet-sand with successive grits, feathering out the patch into the surrounding area and finally buff it. If you have a power buffer, sanding up to about 600-800 grit is usually plenty. If you will be buffing by hand, working your way through the grits up to about 2000 grit is needed to really get nice gloss. A google search for gel coat repair will usually get you several step-by-step articles with photos which go through the process. This type of repair will work on small spots, up to about the size of a dinner plate. If you have bigger areas to fix, you really should have real spray equipment and power sanders to do the job efficiently. It's absolutely possible to re-gel-coat an entire hull, but it's an awful lot of work and isn't a job for amateurs.
Alternate Fix - full hull with paint finish - (if more than just a couple small areas are damaged, this would be more efficient). Again, sand the gel coat fairly heavily and down deep into any cracks. Then using a foam roller, roll on a minimum of three coats of epoxy resin (five or six thin coats would be better). You need about 10 mils of thickness to give you some cushion for sanding. Once the resin's cured, get out the sander (random orbit is the easiest) and sand it smooth using 80-100 grit paper. That leaves a pretty nice tooth for the enamel that will come next. You can use primer if you want, but it really doesn't do much over epoxy. Paint the hull with two or three coats of enamel and you're done.
Unfortunately, paint won't really fill spider cracks. Most of it just sits there, bridging the crack, which will then start to show through, so if you really want to get rid of them they need to be sanded down pretty far and the area filled with some sort of resin, gel coat or resin/filler powder mixture. Any way you slice it, it's a fair amount of work, so make sure the boat's worth it to you before diving in. Repairing one area also doesn't make the hull immune from future cracks elsewhere. Epoxy coating is less prone to allowing future cracking than just spot-patching the bad areas, but if the boat has thick gel coat, it still may eventually crack to some extent and the thin epoxy coating may not be able to stop it.
No canoe pix handy, but this is a fiberglass sailboat which had lots of spider cracks and some gel coat blisters. I re-coated, filled and painted as mentioned above using a roller to apply resin and paint and a $50 Porter Cable random orbit for the sanding -all done out in the driveway.
and had coated the gunwales in some kind of Marine grade varnish.
