The primary goal in butt welding a panel is to replace the damaged area (patch panel repair) such that when completed the weld area can be planished and dressed where it is not discernible from the adjacent metal. We don't always get it perfect, but this should be the goal. Here is a good tutorial by Randy Ferguson where a patch is installed with no filler whatsoever needed.
http://fergusoncoachbuilding.blogspot.com/ As far as lapped panels, where even present day manufacturers use lapped panels and adhesives in joining panels, on
exterior sheet metal that will be visible after finishing, you will only find the laps in areas such as the tops of the roof and any overlap is also in a channel where it can be covered by a flowable seam sealer. They
do not use it in the flanging and overlapping of an exposed exterior panel if it needs to be finished as an "invisible" seam. Plain and simple, the reason is that any attempts to step flange a panel and join to another (whether using adhesives or welding or ??), and finish it off as an "invisible" repair, will result in a ghost line visible in the paint. This occurs as the thicker layers will have a slower expansion rate than that of the single thickness. The area in the middle of these two differing expansion rates will cause a ghost line to show through any filler and paint used to finish off the seam, as the vehicle is exposed to varying temps. Anyone looking at your work will see exactly where the repair was made.
Welding will result in distortion as it shrinks during cooling whether you have a butt weld or a lapped flange. Planishing the welds will help to stretch out this area to restore the original shape. The big difference is it is much easier to planish a butt weld to remedy this distortion than trying to planish a lapped flange. Having one thickness on one side of the weld and two thicknesses on the other is not conducive to planishing and therefore is more difficult to metal finish out any distortion from welding. This locks you into the necessity for filler, and undoubtedly, the ghost lines in your finish.
