Q: My parts are turning black when plating. What's wrong?
A: Commonly this is from zinc or cadmium contamination. If your part has been previously zinc or cad plated, don't sandblast or grind off the old plating. This only impregnates the zinc into the metal, which then reacts with the plating solution. Instead, use a solution of 2 parts muriatic acid : 1 part water to dissolve the old plating off.