you are correct. bondo sucks. dynalite is better. but for what you want to do, make absolutely positive the bumpers fenders and skirts are as secured as you can get them, then use fiberglass reinforced bondo. its the green stuff. also, if you have a light colored car use a light colored creme hardner and the car is dark use a dark hardner. use 80 grit paper to rough every surface that you will be using the filler on, use a wax and grease remover to make it nice and clean after sanding. make straight smooth wipes, the smoother they the less sanding you will have to do. after that you will have to have a bit of a sculpters to sand everything smooth. use 80 grit to get everything as smooth as you can, then go over it all with 320, then 500 grit. lay on several coats of primer. it will help smooth things out further and fill any small scratches that you may have missed. and the most important tip i can give you, is keep everything as clean as you possibly can (i mean dirt free, not good lookin) and dirt, grease or oil will cause bubbles to form over time, or worse case it will break the bonds and cause cracks to form, also im sure you already know paint wont stick to dirt, grease or oil. take your time. i cant stress that enough. spend atleast a week to get everything right. one tiny mistake WILL show through on the final paint job. also make sure that when you mix the filler with the hardner that there are no bands or streaks in it. it MUST be mixed as well as you can possibly get it.
for my credits, im the manager of goldsby paint & body, and have 16 years experience.