i thought this might be interesting to the debate here. i shortened it to the relevent parts:
Butt welding is no way to install a panel, it goes against every rule of modern panel sectioning as outlined by all the manufactures, and I-car. Even if you section it through the window frames, butt welding is an unsafe procedure. (snip)
If you don't mind I'd like to ask you to elaborate further on why lap welding vs butt welding replacement panels. I see it done all the time on shows like Overhaulin, American Hot Rod, and in how-to mag articles and I see them lap some too.
If you do this with 2 flat pieces of sheet, then dress the weld down, the weld is easily broken or torn apart when bent. That is why the flanger is best. The metal seam should be completely welded together, with 1/2 inch seam welds. I also will do plug welds every couple of inches, that is just a 5/16 hole drilled into the new panel, abut 1/8 or so below the lip. This adds extra strength. The reason you see guys butt welding on cars on TV shows is just for speed, or for show vehicles that the guys don't want any seams showing in the trunk, or pure ignorance of what heat does to sheet metal. Also, sometimes it's impossible, or impractical, to flange the existing panel to accept a patch. The thinking is, that the lap is a place for rust to start, which is true. I look at it from a stand point of strength, because I'm a collision repair expert. I'll sacrifice a little longevity to gain strength. The vehicle surviving an accident in the area I repaired is incredibly important to me, from a personal and legal stand point. If I butt welded a panel in, and another vehicle blasts through the panel like it isn't there, then I jeopardized that person's safety. That is why I don't like butt welds, the heat permanently weakens the metal, no matter how much skill was involved in the repair. According to I-car, even my flanging technique isn't acceptable on rocker splices, sail panel splices, and splicing a panel through the windows. Much less a butt weld! I-car says to fit the pieces up, and trim them like you are going to do a butt weld, but leave a 1/16 gap between the panels. Then, you take a 2 inch wide piece of sheet metal from the old panel, fit it up and under the existing panel. Then you plug weld it into place, leaving 1 inch sticking down that the new panel laps on to. Then, you drill 5/ 16 plug weld holes into the new panel, every 1-1/2 inches. The new panel is fitted, and the panel is attached at the seam with plug welds. Then, the seam is closed with half inch stitch welds. It's incredibly strong, but prone to catching water. So, it needs undercoated with paraffin based under coat. Sound familiar? But, when I send a young woman and her little children down the drive, I know that if they get hit, my repair will protect them, and the car will react in the way the manufacturer intended.
22 year veteran, 2 years of vocational training. A.S.E. certified Master collision repair/refinish technician. I-CAR certified. 17 yrs. GM experience. I can answer most questions related to automotive body repairs, ranging from small scratch repairs all the way up to the most extensive collision repairs.