I suspect a lot of the panel weld seams you have seen is because the shops failed to remove the protective coating the patch panel came with. A lot of shops think that protective coating will "burn away" in the weld process, it does indeed burn away, but it usually leaves a residue that causes contamination at the weld joint. That contamination causes weld porosity which leaves holes in the weld line which is often just ground off and left as is. The issue is compounded when the weld joint is a butt weld, there is a better chance of the holes to exist. Then they cover the holes with filler to create the perfect rust line at the weld seam.

A poorly welded panel joint will show up on every poorly welded panel at some point. There have been a lot of poorly welded panel joints over the years, probably as many poorly welded joints as there has been good weld joints. You will eventually see the bad joints, but the good joints will probably never be exposed. Unless you know the panel has been patched, and the good welded joint doesn't ever show, you won't know it was ever patched, so you only see the bad joints. Gene