I recently bought a gallon of (tinted) Gelcoat for about $60 to $70 US. Add in some thinner (styrene) and the PVA needed to top coat it and you're at about $100 US...plenty to do a whole 20' boat. I thin the gelcoat anywhere from 40 to 60% with styrene (PLEASE where breathing aparatus - this stuff turns soft tissue to goo). I've listened to plenty of advice - some yelled loudly to use the wax additive for the air dry but I've had miserable results with it and I've done a lot of sampling and paid a hefty fine for an incomplete cure. I recommend, instead, to spray a layer of PVA over the uncured and freshly sprayed gelcoat to cure it (using air-inhibited gelcoat of course adn allowing enough time for the styrene thinner to flash off - usually about 10 to 20 minutes). It's very dependable doing it this way.

And oh yeah...I don't have any direct experience but there are those that say spraying gel coat over epoxy will never work (some say it works OK).