Muriatic acid works great just the fumes.
I've tried lot's of potions , etchant also works , Nickel doesn't seem to like constant temp and humidity change and to be honnest if you can just get it started to dull down it goes of its own accord pretty quickly.
There are however lot's of companies selling aged hardware , it's not dificult to find it on the web. The Les Paul Forum has a million threads about ageing hardware.