Once the plating is gone, it's gone. The only way to fix it is to have it replated, ideally all at once (i.e., strip the old and then replace it). Costly, takes the horn out of commission, and (in the end) only puts you in a position to have to do it again down the road.
Some of us have real problems with plating. In my case, straight up nickel over nickel silver keys come up frosted within a week, and through the plating within a couple of years, no matter how carefully the horn is wiped down subsequent to each playing. Only by going to silver did I manage to break that cycle, and since that time they've stayed unfrosted.
Others have the same problem with silver (as mentioned above). For you, the nickel standard plating may be the better choice.
This is partially a function of your own chemistry, and partially that of environment. I've written elsewhere of how my Yamaha YBS 62 turned color (in part; posts only) when exposed to our chemical-laden Ship Channel atmosphere a few years back. Similarly, an unprotected silver plated horn (one kept without anti-tarnish paper) can go black in a very short time. Not too much you can do in those situations.
As for my skin problems, I wash up before, during breaks in performance, and afterwards, just in case that will help. It costs nothing to do and keeps the skin oil and salt levels down overall.
If you are one with acid or reactive skin, that's about all you can do. I'm quite familiar with barrier creams (having worked for OSHA for just under 40 years) but not one of the many that I tested was suitable for a musician, much less for a clarinet player. Ditto very good latex gloves - they just don't work well enough to suit me.
I did forget gold plate. Gold doesn't react with much, but the alloy metals used with the gold (which is never "pure" gold) still does. On my old Conn horn, once graced with gold plate but now resplendent in silver, there was abrasive wear of the gold plating at the left side of the rear of the horn, but there was also "chemical" wear (i.e., the plating was gone but without any scratches showing contact with an abrasive surface) under where the players' hands had rested next to the right side of the thumb hook. So, even gold (and I would imagine platinum) would not be completely immune.
