It gets quite frustrating. Archaeologists, expert numismatists, metal detectorists usually learn which coins may benefit from cleaning, and how to do it. These people will often use an ultrasonic tank. This is not a magic device, it merely uses ultrasound waves to agitate the liquid in which the coins are immersed. This will often be soapy water, and nothing more.
Sound waves are gentler than a soft brush.
Many people now own miniature domestic ultrasonic tanks, sold for cleaning jewellery. Before you are tempted to clean your coins in such a tank, remember the advice we have repeatedly give above, if you are unsure - do not clean coins. Copper and bronze coins will almost always end up looking a brassy unpleasant colour after cleaning. Even gold coins can sometimes be ruined by removing a rich tone which has built up over a long period of time. As we have made clear above, one of the easiest way to spoil coins is to scratch or abrade their surfaces by using an abrasive.