To clarify ; was told by someone some time ago ... that mig welding , even done expertly, will show in transverse as well as longitudinal seams due to the density /hardness of the mig weld process{.if I remember correctly} due to different expansion rates of expansion weld vs adjacent sheet metal , even ever so slight .example black car hot day in the sun will show a shimmmer ??
If done correctly, I've never seen this happen before. I did body work for a living for a few years and have done it on the side for years since.
I did a bunch of patches on a truck that I recently saw at a car show in the heat, I did not see one spot where the patches are and I know exactly where the patches are.
Have I ever seen this happen, yes but, based on the quality of the rest of the job, I would venture to say that the patch was done incorrectly. Never have I seen this with any patch that was done by someone that I know, that also knows what they're doing.
With that said, I believe what you're describing is possible under the right circumstances, just not probable. Years ago, I did a bunch of work on SCCA race cars, which included a lot of fiberglass fender flares attached to steel bodies, etc. With those, you could see some lines in the sun where the different materials met, which was due to what you're describing. But, again, that was with steel, fiber glass and body filler.
