About once a year I give my cast iron a real scrubbing and conditioning. For this I do use soap and wear some good cleaning gloves and a full front apron I personally like the japanese style apron. What I do is I use a scouring pad pot scrubber that is made of stainless steel. You can probably find the same kind of scouring pad at your favorite local store. Fill your sink with hot soapy water, put on the apron and gloves, grab a scouring pad and the target pan and scrub. Scrub like crazy all over the pan. I start with the inside up the sides of the insides since they get slightly scaly. Then go after the outside and for this cleaning you want to concentrate a majority of your scrubbing to the outside of the pan. Especially around the outside where the cast iron scales. This is a messy job and you will get black splatter all over. If you have a two sided sink just scrub the pan in the well on the dry side and just dip the scouring pad to collect soapy water as you make your way all over the outside of the pan. Just a word of warning…you will never get all the scales off. The scaling is a natural process that happens with cast iron when it heats and cools. My suggestion is to scrub until you get tired of it or you just decide that it is enough scrubbing. Once this is done then rinse, rinse, rinse and dry the pan with a rag or paper towels that you can throw away. Then get out some oil to coat the inside and the outside of the pan. Then put the pan on the stove and heat it up to almost smoking, turn off the stove and let it cool. You are done with the once a year cleaning.
Recently I was visiting my daughter and she has started using cast iron. I looked at her pan and the bottom outside has a little bit of rust on it. When I asked her about it she said that she didn’t like putting oil on the outside because it would smoke. After she told me this I realized she is putting too much oil on the outside. Folks the outside of the pan only needs a very light coating. In fact I will take a paper towel and do the inside and then just a quick wipe on the outside. Very little and since I don’t actually wash the pan I use salt to clean my cast iron, read about it click here. I don’t have to do it very often and the coating will just stay on the pan without smoking and protects the outside from rust.