Seriously, Bondo is one of the all-time greatest materials ever for filling problems in models - It is catalytic so it will always cure fast and reliably (assuming you mix it properly), it is fast so you can go back and sand it down again in only 20 minutes or so, and it sticks well to itself and other materials, so you can patch again and again to build up larger areas. The drawback is the smell and probable carcinogenic effects, please use it in a ventilated area:1) Mix as directed on a scrap of disposable wood (paper plate, etc.)
2) Spread with a plastic scraper, chisel, popsicle stick...
3) When it starts to 'kick', or gets grainy suddenly, stop working on it.
4) After 3 to 5 minutes, go back with a razor blade and slice off the high spots before it gets too hard.
5) After another 15 minutes you can sand and paint, even though it seems a little tacky still.
good luck/pRC