Let your pan dry before getting to this bit, or you can try scrubbing in the solution as well to pick up any debris. The main idea to this step is to get rid of ALL the rust that has eaten away at your pan. Scrub like mad, until you can't make out the region of damage on your pan. I used a copper scrubber, but depending on the severity of damage, you can use anything from rock salt for spot touches to a drill mounted metal scour for those heavy duty jobs. Try to be as thorough as possible- you've already lost the seasoning in those patches, so scrub to your heart's content.