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.
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.
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...
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