Are you talking about zinc-die cast parts that are plated, or steel parts that are zinc coated? Any metal polish will shine up a zinc plating over steel, but it is very thin and you will easily cut through. Since zinc plated parts are not polished, it isn't going to work. If you want to get them re-zinc plated, it is a lot cheaper than if you were getting them chromed. If you want to polish up your pitted old zinc die cast, then use the search function with words like: diecast, zinc, pot metal, polishing, or whatever might work. This has been discussed on here before to great lenghts.
Even if you do, it needs to be clear-coated or it will just oxidize again. If you choose to do this anyway, jewelers rouge works pretty well.
The Jalopy Journal
