I just bought Jax Green Patina. May this be applied to hardware that already has a nickel finish on it? Or, should I use my Dremel grinding wheel to try to remove the nickel plating first?
You'd have to read the instructions. Normally, green patinas work best on copper and, to a lesser extent, brass.
Jax has a special one for nickel, which is black, and it works quite well.
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