My kids are very excited to do pressed pennies during our upcoming trip. We have lots of pre-1982 pennies to use and have been working on cleaning them up a bit. We've tried vinegar, hot sauce and Never Dull, and I thought we had pretty good success with each of those. But with all of those methods, the pennies went right back to looking dirty after a few days. Any suggestions? If they really can't get cleaner, I'm okay with it. I just wondered if I was doing something wrong?