Only dip coins as a last resort!
Coin shops do sell dipping solutions.
dip for 5 to 10 seconds, next Rinse in warm water to neutralize acid & then immediately dip and rinse with the purest alcohol to drive off the water,(Water, finger print acids-chemically etch the polished surface of a coin -causing tarnish) the higher alcohol content eliminates possiblity of water spotting - when doing the final alcohol rinse a blow dryer set up next to the work area and a safe distance to avoid fire! and up and running is used to heat and dry coin, go straight from alcohol to the dryer in one swift careful movement.. most important of all is no rubbing with paper towel/ cleanex/ cloth etc. cuz it all scratches.. the end result is sometimes a nicer coin, but never as good as a not cleaned coin! dipping etches the surface, etched surfaces are not as lusterous/shiny as a non etched polished surface. So only dip if the coin is down right ugly- would I recommend cleaning cuz the average person has not the expierience handing metals.
My back ground is in Metals, Metalsmithing since I was 15yrs old and I am familiar with many techniques regarding the making and use of most common metals. I have poured Aluminum, Lead, Silver, Gold, Iron , and Steel, the last two on the list Iron & Steel being the mainstay of my career. My final position (Manufacturing Developement Technician) and head of my Metallography lab for 10yrs required use and knowledge of current metallography, grinding , polishing , etching, techniques for microscopic analysis of ferrous and nonferrous metals.
hope this sheds some useable information on the subject..
Gabe