Mike,
For plug, molds, and such, I'd go visit your local automotive paint store (even NAPA) and pick up their cheaper brand of primer. I usually pick up the gallon cans and buy the heaviest one. You can spray it or brush it on since about 95% or more will be sanded off anyway. I've used the Evercoat brand, "Dura Build" gray Acrylic Surfacer primer that's "high solids". Works great on my model projects and you can brush or spray it on over West Systems epoxy glass work in preparation for paint. I believe it was around $30/gallon. You can also go with the Epoxy primers which are higher priced.

There is also the Duratec primers that are specifically designed for moldmaking and can be polished to a mirror finish. You can order the gallon online or at a local boat or f-glass/plastics shop.