Also don't use any metal polish, no matter how fine it says it is unless you just absolutely have no choice.
Metal polishes are abrasive and each time you use it, you're removing a thin layer of the nickel.
You can polish right through the nickel.
Use a metal polish only if the finish has discoloration, NOT to attempt to polish out scratches, even fine ones.
The best way to care for a nickel gun is to keep a thin layer of a rust preventing lube like CLP Breakfree on it or use a good wax like Johnson's Paste Wax or Renaissance Museum wax.
NO CAR WAX.