Well, it all depends on what you are trying to do.
I imagine you want to do some "custom-moldings" .. for this, I'd use some short-hair fiberglass to build up a base layer, then some "body filler" to patch up all the low-spots. From there, a spray-sandable primer knocks HOURS off your sanding time ..
aside from that, pretty easy. Here's a place to go for more fiberglassing info:
Look in the "FORUMS"