[Lukas]

Dear friends, I tried to search everywhere, but still without result.
I have some metal components from my loved older electric guitar, which are die cast zinc or (according to manufacturer) hardened zinc. This zinc material is clasiccaly copper-plated and nickel-plated on top.
There was a corrosion from sweat-acids caused by previous owners, which penetrated through plating and created bubble effect. I sanded this corrosion and buffed the surface. Now, on some tiny places, there is exposed zinc, somewhere little of copper, the rest is mostly nickel.
My problem is, that lots of electroplating factories I asked in my country - cannot guarantee stripping down to the basic zinc. Plating of clean material is ok, but they say that stripping without damage is lottery.
Could somebody help me, if there is some solution, which can strip the copper and nickel, leaving zinc intact?
I found info about lime sulfur (calcium polysulfide) and tried it on one part I have extra (sanded a little bit). Works great, this liquid is dissolving the copper leaving zinc intact, but makes nothing with nickel...
I am amateur about chemistry, but good overall technician with tools, never risking and keeping safety on workplace.
Is there anything I can do, to strip these platings down in common utility-room?
Please, excuse my mistakes in English, I am still learning.
Thank you for any answer in advance and many greetings from Czech Republic.
Lukas.

[Zyklonb]

I'll tell you the same thing I did last time.
My advice, dissolve the nickel in a non-oxidizing acid - hydrochloric acid is likely the best. This will take a very long time, other acids do it faster, but either they're harder to get, or they will oxidize the copper, which you don't want.
Be sure that the entire nickel layer is gone. [Make sure to dispose of the nickel chloride solution safely, nickel salts are very toxic and can be carcinogenic]

Next, dissolve the copper layer in calcium polysulfides solution, as you found your self.
The reason I suggest this procedure, is because I don't know of any acid that will dissolve nickel and copper, but not zinc. Zinc dissolves much easier in acids then either of the other metals. So as soon as the layers of Ni and Cu have even one small hole, the zinc will be destroyed while still encased mostly. That would be bad obviously.
Also, Make absolute certain to wash off ALL acid before treating with calcium polysulfide solution. It reacts with even dilute acid to produce extremely toxic hydrogen sulfide gas! If you smell rotten eggs, run!
Hope this helps.
[EDIT] Typo, and:
Well, I guess nickel (II) chloride isn't quite as toxic as I thought, still not something to play around with, per say. Just wear gloves, splash goggles and try not to drink the solution.