I had to do a lot of welding on my rust bucket and I'm getting close to being done with the welder. I've butt welded all external body panels. I also used the strip method described by jumpinjan to back up a joint in areas where apprearance on the backside wasn't a factor, like the floor that will be undercoated anyway or a fender patch that will be closed in. I've also used the joggler pliers to build in the step. All depended on the area and access I had. For an external panel with a curve, the joggler might further distort the metal and your body guy will have more work.
I lap welded most of the floor panels and trunk panels (had to replace every single piece of floor/trunk/baggage shelf). Remind me why I started this project? I used a combo of clecos and sheet metal screws to close the lap gap. Some areas, I just welded an edge seam especially around the widened trans tunnel. Others, I drilled holes in the top piece of the lap and spot welded going back to fill the holes used by screws/clecos. I also used weld through primer on closed/lapped areas, but don't feel I got as clean a weld. Had some splatter.
For the butt welds, I left a very small gap between the parts for the filler to flow into and got a better weld than if the parts fit tight together. Where possible, I used a high intesity light behind the piece to check my work. Once ground smooth, I occasionally found a pin hole to close that I would have missed otherwise.
Choose a small diameter wire! Copper certainly helps to back up a butt weld or behind a hole you want to close as bashby noted to keep blowout to a minimum. Copper dissapates heat quickly. I mady my own by going to the plumbing store, getting some copper pipe, smashing the end flat and shaping it to fit the area I was working. 2cts.
Good luck and let us know how its going.