I know it can dry out. Maybe you used too much or too little of the cream hardener? A better compound is Feathercoat or Formula 26 I think it was called. It's a higher grade filler with less shrinkage and is smoother. It adhears great to acrylic and gell coat. You use to beable to get it it Home Depot and Lowes. If not, do to a auto body and paint supply. They will have it and oother fillers. Tell them what you want to do with it and they can help you.
Mark