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Follow the intsructions in the instructable, not in the muriatic acid... So no, do not dilute
Let it dry up or neutralize with baking soda and take it to your local hazardous material center. You never want to pour down the sink or throw in the yard, the dissolved copper is highly toxic to fish and other animals.
Works on brass, but the etchant gets gunky, so I use a separate container for brass
I have used it to etch aluminum... It causes a thermal reaction and gets pretty warm and bubbly, and etched quickly. It was already used etchant. I'd do it outside of your going to try it, fumes a lot too.
Stop using Ferric Chloride etchant! (A better etching solution.)