There are two more solutions, and in my opinion more reliable options, for curing gelcoat on an exterior surface.

1) Duratec gelcoat additive thins the gelcoat slightly so it will spray a little better and it also acts to promote curing of the gelcoat in open air - you don't need any additional additives to spray and cure. Reportedly the duratec cures the gelcoat harder than it would on it's own.

2) top coat with PVA (poly-vinyl alcohol). After you spray the gelcoat, you can spray a layer of PVA ontop of it to seal it from the air and allow it to cure. The PVA forms a "saran-wrap" esque coating that will peel or wash off easily once everything is cured. It is very water soluable and I use it all the time when I'm molding things to prevent resin from sticking to my molds.