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 ??
Again, no. Whoever told you that doesn't know schitt from canned Rhubarb.
Grind the weld flush, primer, filler, skim coat, primer, and then paint covers the metal. Done properly the weld is invisible after grinding it flush.
