I poured straight vinegar in mine, heated to high, then held down the steam button till all drained. Did this about three times before no more residue came out. Rinsed with clear water about three times, also.
One thing I never see mentioned is that we should use Distilled Water in our irons as that does not have the minerals in it that tap water does. Seems to me anyone that has a water softener would not qualify either.
Mom and DG always used DW or a filter for the water and never had problems with their irons.