Most keypads I have opened have gold plated contacts and conductive rubber 'buttons', In this case the gold plating is replaced by a hard black conducting substance, I take your point about rubbing off (or removing with solvents) this black substance but the PCB making process would have to allow for some cleaning of the surface without damage. Also it is not recommended to do anything to the Remote if it is already working O.K. So it's just a case of 'How bad do the keys get before you take action'If you rub it off they never work well again. Sometimes not at all. Hence my note above. It is not a good idea to use any sort of rubbing on them either dry or wet in my experience for this reason.