: What your uncle had applied was not really a gel coat although it took the
: place of the gelcoat. Gelcoat is applied as the first layer to the
: interior of the mold that a fiberglass boat is made in.

Normally, Gel Coat cures only in the absence of air, as is the case when it's used in a female mold for making fiberglass boats. However, Gel coat can indeed be applied to the outside of a hull. To do so requires special activators to be added in order for the gelcoat to cure properly. Alternatively, a wax perparation can be sprayed over the Gel Coat to seal it from the air.