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 ??
My buddy is a mechanical engineer. He absolutely swears that on dark cars you will eventually see the mig lines if you use a mig. I have personally never experienced it.
