[just dreamin]

My thoughts, for what they are worth. Use a shielding gas as well as the flux core wire and give the tank coating a miss as it is another potential problem area if it doesn't bond. The rust concern although valid, is not a going to be a greater issue than what is there now, and should not grow as it is only caused by the welding heat that alloys the two metals.
My experience is only personal as I am not a certified welder, so feel free to disregard it.