If you go the route of using a polymer based filler like bondo, I'd say use Evercoat. Its what the pros use, much easier to sand and apply.
My tips for using filler of any kind is apply it generously, just slap that stuff on there. Don't try to spread it on there perfectly and get a finished surface, its not going to happen. Once that stuff is on there, get some 40grit sand paper and go at it. Don't be afraid to chop it up. Once you "shape" the piece with your large grit then you up the grit 100, 150, 220, 300... til the surface is smoooooth. If 80% of your filler isn't on the ground after shaping then your not doing it right.
Also when starting, gouge/scratch the hell out of the surface your bonding to. If ts smooth, it will have nothing to bond to.