Short strand fibreglass bondo is a very good option as well... But you have to work fast, because once it's cured, sanding becomes a bitch with small scale stuff. I've used it with a couple of models and it's brilliant stuff. It cures at a speed that gives you a change to sculpt and then cut off bits while still on the soft side- then sand.
I used it on my Klingon ship kitbash.