If you get a handful of coffee grounds, one of the first things you will notice is how coarse they are. Now if you happen to get some used coffee grounds, then dry them under the sun, you make them even finer, and coarser. Those tiny granules are sharp enough to be used as sandpaper, so making the jump to a stainless steel pot is not difficult.
Once you have used the pot, sprinkle in a handful of used grounds. They don’t need to be dry to be effective (but dry grounds are the best). I also find that using spent grounds along with some liquid dregs straight out of my copper Turkish coffee pot works well.