This should only be used on modern coins or coins that have little value, as collectors of old coins will tell you they rather their coins dirty, and is going to be worth a lot more to a collector as it is than if you clean it.
My advise is if you don't know the coins value leave it alone.
Shown in the photos are some before and after shots of coins Ive found, the 1 and 2 cent pieces are obsolete in Australia (from 1994) and are now starting to be collectable. They can be worth up to $3 each depending on condition and year, which is a 30,000% increase in value, sounds great but is still only 3 bucks.
Also shown is a pre-decimal six pence which are 50% silver again worth about $3. The other coins are current and can be spent.