ATTENTION - PLEASE READ:
All new cast-iron pots and skillets have a protective coating on them which must be removed. American companies use a special food-safe wax; imports are covered with a water-soluble shellac. In either case, scrub the item with a scouring pad, using soap and the hottest tap water you can stand.
I am assuming that a clean pan should not still have brown stuff coming away from the surface. It's not rust and not visible. Dry cloth comes away clean.