Black Base Coat Under Aluminum - Why? Base Coats in General Too

I think Alclad also recommends a base coat of gloss or flat black, depending on the surface finish you are after - yes, a black finish under an aluminum top coat will add depth to it. I used to use flat black under Metalizer myself, until I discovered a better way of buffing it out as applied to bare plastic.

Depending on the shade of metallic you are using, black, white, or grey make good undercoats. If you are looking for a shiny surface, use a gloss undercoat - Alclad specifically asks for an acrylic gloss undercoat, I recall - for a "mirror finish". If you'd like the surface to be dull, semi-dull, or less "mirror like", use a flat or semi-gloss undercoat stepping up from black to grey to white.

I also went though a phase of undercoating/priming all of my model aircraft with rattle-can silver before applying a final color coat from an airbrush. I found that this not only showed up sanding flaws REAL well, but added some depth under a light color (like ghost greys) and also provided a nice way to produce chipping in the final finish...silver is also a pretty nice base coat under white (did that on my 1/72 XB-70). I could use less color paint this way, too.

But I don't use any sort of primer coat these days; I get Metalizer to work best on bare plastic, and I've gone back to doing the same with the rest of my color enamels as well.

Makes it look like metal, rather than paint...

any black gloss or flat, depending on the finish you want.

Actually, you can get quite interesting results by using other colours, like deep blue, dark red, dark green or shades of gray.

No, you want a GLOSS black, as glossy as possible. I have applied chrome and aluminum alclads over other colors, but the gloss black indeed does make for the shiniest resulting aluminum finish. If you want a shaded metal finish you can put down several colors and states of gloss before the Alclad. Sure don't know WHY the black works so well, but it does. The fact that a glossy finish creates the shiniest results is not surprising, just the color.

If the NMF is Alclad then the reason for the base coats is necessary to protect the plastic. It is a lacquer based product and could potentially craze the plastic if not protected with a primer.

Actually, flat works very well if what you want is an anodized looking sort of finish vise a polished one.

Black works because it adds depth and aids reflectance - just like on the back of a mirror. Which is flat black, generally speaking...