That powder stuff is “bauxite” (pronounced Boxsite) it is aluminum hydroxides, which is the principal ore of aluminum. No matter what you do, eventually aluminum is going to return to its natural state which is powder. I wash my machine down with 10% bleach water and soap solution (by hand ), I allow it to dry on a towel on the table for a day or two, then I will give it a give it a light coat of food grade spray oil (I think it is mineral oil) before I put it up in one of those plastic tube form Wal-Mart. Usually you will see Boxsite if there is a presence of moisture or salt. Even though you may washed and dried it, there may be moisture in the pores of the metal. Maybe it is the humidity in the area of where you live doing it. It may just be me, but I think drying it in the dishwasher would get it warm enough to get all the moisture out of the pores. I have had a spot or two on pots in the past and I used a Scotch bright pad on it until I removed any problems. Hope this helps. SB