A lap weld is more likely to show the seam as a ghost line. Two layers (where the overlap occurs) is going to have a slower expansion rate when heating up (sunlight) and a slower contraction rate when cooling off in the evening than the single layer adjacent to the repair. This difference in expansion rates will cause a ghost line to show exactly where the repair occurred. A butt welded seam that has been planished and dressed afterward will give better results.. Here's a video that shows a ghost line from a lap seam in a tailgate repair.
