Cast iron is porous. Leave the crust on the outside on. That is the grease that bled through the porous iron over who knows how long. When Mom got her first stove with cleaning cycle she ran Grand Ma's skillets that had passed won from her Mom 1857-1933. It burned off all the old grease inside and out. Ruined the conditioning..and if you don't get oil on it while it is still hot it rust not just on the surface but int the metal. Those skillets were less porous than the new skillets manufactured deliberately to be porous. Grand Ma cleaned the inside with salt and a cloth and put a little pork fat back in and rubbed it around. Bacon fat has salt, which likes metal and water. I still can taste r imagine it of fried chicken and everything else in those old skillets. Not much stuck and if it did you cleaned it off.