When polishing nickel, keep this in mind.
A scratch in nickel can only be removed by removing the nickel around the scratch.
This is similar to "removing" a scratch in wood by sanding down the wood around the scratch until you reach the bottom of the scratch.

In the case of nickel, you're literally abrading off the plating to "remove" the scratch, and this thins the plating.
Polish a little too hard, too long, or too often and you'll polish the plating right off.

EASY DOES IT, and not too often.