Yes, dirt on the pennies will act as an insulator and keep the Terminator from being as effective. You should clean the pennies whenever they get noticably dirty. They don’t have to be shiny bright, just reasonably clean.
As you use the Terminator and clean the pennies you will notice that you can’t get them looking shiny bright. A dark color won’t cause any problems, only when you get a build-up of dirt or “gunk” will the effectiveness decrease.
Don’t clean the pennies so much that you wear them out or knock them loose from the box.
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