I know this has been discussed a million times but the search tool isn't pulling up anything helpful. Being that my oldest is only 3.5 I don't think we'll do more than a few penny pressings but it would be fun to do a few. What are the most ideal penny years for this? Something before a certain year in the 1980's right? And all my pennies from prior to an 80's date are pretty dark and dingy. Anyone know a good way to clean that up a bit?