No getting around it,, fiberglass just plain sucks!
Just about any body filler these days is flexible. The trick is not to get it too thick.
One of the problems with fiberglass is that no matter how many time you block it to get it strait, the next day it will be wavey. No getting around it. Fiberglass just moves around.
Just bondo it like anything else.
As for primer, you want to use a polyester primer. It actually uses fiberglass resin as a hardner. And the best thing about a polyester primer is that it doesn't shrink. It's the only primer that doesn't. All urethane, laquer, ect, primers will shrink.
And it filles like crazy! A bodymans best friend!
Craig