Quarter panels, let's cut em off and put new ones on.

One thing I learned about butt welding that not too many people mention is to make sure the edges are clean and square after you've done your cutting. You may wind up cutting the panels with a variety of tools that can leave burrs or beveled edges. The burrs will heat up and cause you to blow through when welding and beveled edges will also cause you to blow through because you are now welding an even thinner piece of what was already a thin piece of metal.

I clean mine up on the bench grinder at the very end and then look down the cut part and make sure it's nice and flat and square all the way down. It's a little more difficult to do on the car side since you cant get it on the bench grinder but take your time and prep the edges and it'll make welding a whole lot easier.