Need Any Welding advice?

So what is the best way to clean inside a body seam for welding?Its not uncommon for me to have to weld on a body seam on 40+ year old dirty,rusty,tin on a car body.I have tried everything I can think of but sometimes I just have to turn up the heat on my MIG and burn the crap out and pray that the weld holds...looks be damned.