You have got the crud off of the penny, now is time perhaps to use some baking soda paste to rub it down. It looks pretty worn so it is not a high dollar coin. Just make a paste and rub with your fingers...if that does not do it then get a soft fine fiber brass brush and brush it...Brass is soft enough that it will not scratch the copper. KT has one that He used to use on his suede shoes for cleaning. Let us know how it does!