Working at the same body shop for 13 years I got to see some of the same cars. Meaning, I got to repair several cars that we had already fixed. I replaced a rear body in a car that we had previously replaced 3 years earlier. Our standard operating procedure for repair always included weld thru primer, replacement of seam sealers and undercoatings and we etched and primed all panels as if it were an exterior panel. In other words, we didn’t seam seal or undercoat over bare metal or e-coat. The rear body weld joints were as clean and rust free as a new car. The lap joint is not the enemy, lack of preparation is. Even a but weld can rust if left to the elements.