mig is the only way to go on body panels. Give it a little zap then move to anouther area spreading out the heat as much as possible, trust me your gonna have to use some body filler no matter what. I worked in a bodyshop through highschool and this is how they do it. Try to avoid tig on body panels if at all possible as for cleaning the backside try sand blasting it might be a pain but if you want a pro job its not always the easiest