The plater will run everything through an acid bath to remove any pre existing plating, oil, dirt etc. but if the components are rusty they should be blasted first. Zinc plating will not take well to rust from what I've seen in the past. The blasting can give a rough surface which is not great for finish so polishing
any surface defects out after is a good idea. The plating will not cover up anything. What you see is what you get. Ive hand polished what I could before
the process. As shown below. Final results were great.
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