Most of us learn by doing so it looks like your already on your way to becoming a pro. You didn't mention what size wire your using? I use .023 in my mig. I've found that the tighter the "fit" or "butt" weld area the better the welds turn out. It looks like you've got a handle on welding, spaceing the welds and keeping the heat under control is crucial. Hammering and dolly work are just a part of the process. Bondo will not crack, unless its applied to thick. Good luck with your project.