Undercoating metallics?

Metallics are essentially metallic particles suspended in a transparent/translucent binder/carrier (often it's this medium which is tinted to give them the required colour rather than the metallic particles themselves) and are often not entirely opaque (unless applied very heavily). I think that using a black undercoat simply provides a high-contrast backstop which emphasises a lighter metallic in the case of any "show through".

It's sounds like something cool to try next time I do a metallic finish.

Much depends on which product line you use.

With my products it is not required. You also need to keep in mind that if you are building a model with fine recessed details, the more layers you apply in the painting process, the greater the chance you'll loose them.

I prime with aluminum (my paints of course )...start with with real metal just like the real aircraft.

 

I noticed that a gloss black undercoat made my aluminum paints lay down much smoother...

Until I built that F-86 a couple weeks ago, I'd never undercoated before, and I must say that the difference was rather surprising..