I picked up a bunch of 5.7x28 cases at the range. I don't shoot them but I thought I could sell or trade them. I noticed the first time I tumbled them they seemed to have a coating. The second tumble session removed it. I noticed on another forum that someone was asking for 5.7x28 cases that still had the plastic coating on them. Was I not supposed to remove the coating? Are they scrap now?