Somebody asked me why I removed the EDP before priming once and I told them exactly what you said. A lot of them do have surface rust under there. Just because it's new doesn't mean somebody didn't rub their oily fingers everywhere while it sat waiting to be coated. Plus, it's more to keep the panel from rusting on the warehouse shelf than on your vehicle. I usually strip the area to be welded to bare metal. You can weld through the EDP but it starts to bubble. On something I want to last I'll prime the back with a real 2 part epoxy or urethane. It'll burn when welded but not as bad as "weld through primers" which aren't even waterproof so the rattle cans are basically useless in my opinion.