…...and to show the effects of a ghost line that Kevin spoke of, this video shows a ghost line in the rear tailgate of a wagon that had been repaired using a step flange to attach the patch panel at the bottom.
This occurs because you have two thicknesses of metal to one side of the weld seam, and one to the other side. A single thickness will expand and contract more quickly than two thicknesses. Given enough heating/cooling cycles, this differing expansion/contraction rates will cause a ghost line to appear in your paint finish showing EXACTLY where the repair was made. Using butt welds will help to eliminate this .. Planishing, grinding welds flush, as discussed in the sticky thread help to eliminate it further.
