I use aluminum metalizer as my go to for silver body colors. It's the most accurate representation out there for an in scale silver.
I don't see the point in spraying it over black personally. I suppose it would help you see where you are painting. But part of the reason it says to spray over bare plastic is because a plastic surface is completely smooth. Primer, and paint add texture to the surface and take away the fine grain look of the metalizer.
But if you are going to use it as a body color you obviously need primer for a durable finish. So you need to sand the primer to a perfectly smooth surface. Use 2000,2500, or 3000 grit. 3M makes a 3000 grit sponge that makes it a lot easier than it used to be.
Metalizer can't take masking it will come off. You would need to clear coat it first. And even then Ive had instances where tape will can take a patch of clear coat off and leave the metalizer. Probably because the clear doesn't have much to bite too.
It maybe wise to spray the metalizer, then use metalizer sealer, then clear coat. Though Im pretty sure metalizer sealer is just a flat or semi flat lacquer coat. Don't know for sure though.
Here's an NSX i sprayed with aluminum metalizer, then wetlook clear.
