[Zeke]

The welded metal will indeed rust very quickly and aggressively. The backside needs to be treated much the same as the front, or topside. This means a thorough cleaning followed by protection. POR should work well if worked into the welds completely. You could follow with underseal or SKS stone guard. The race cars I've looked at have just had the welds dressed, cleaned and painted with the same paint used everywhere else on the car.