Hi! If you're like me and sometimes don't care about condition (maybe already sooo toasty; or a non-coin find you've spent a week trying to figure out and just had it so BAM--harsh solution), but just want to read dates to record finds or such--one thing that works great on copper pennies is "Simple Green" brand limescale remover. Or probably any cleaning product that uses the same main chemical (I think it's urea hydrochloride). After a day's soak just about all the patina stuff on top will be gone (well, of course, you should first vigorously scrape as much as possible with a toothpick or something, as long as it's not on the date itself). Some tough things need repeated days. A bottle will last a long time though.
If after the cleaning it turns out to be a 1909-s VDB, well big whoop! If you hadn't cleaned it it would've gone in the trashcan or random unknown date wheaties; and if it needs that much cleaning it's probably worth only a few bucks even for that coin. Big whoop! When in doubt and all else fails, chemicals! And also possibly wire brushes if you still can't read the date or other attributes of whatever you're curious about.
