My experience with fiberglass mould is you'll have to make several small coats of latex to make sure that its cured straight through Its because fiberglass can't take away any moisture like ultracal .
Hope it helped :)
I've always used one layer of prosaid then the colours then finish it of with a couple of layers permawet and then I get no craks just make sure to wash the latex good before starting with prosaid
I just went with one layer on this and yes prosaide is a bit tacky but when u get your colours on it stops beeing tacky. Trust me better to have a tacky start then a crackling finish :)