"Use the airbrush like a paint brush not an aerosol"
"he polished aluminium does not really look "mirror-like" (I don't expect chrome, but still.) Yet, on my scrap-plastic, one spot was really shiny when I tried it first (but it was really heavily coated on the first pass) I used the same gloss black, then why is the difference?"
Paint with many thin even strokes, not just one coat. Build up even coats until you are satisfied, but only one coat at a time or you may get the uneven finish. You want to put on a heavy finish, but in multiple thin coats.
"How do you apply decals to a polished aluminium or a chrome surface?"
Just as you normally do.
"Can you use SolvaSet?"
Yes, but that is strong stuff. I would recommend using MicroSol.
"How does Future look on that surface?"
Looks like a normal gloss finish.
"What kind of colors do you recommend for airplanes?"
I don't use Alcad, so I'm not sure all these colors are available. I recommend aluminum, polished aluminum, steel, burnt metal, and gun metal. I prefer to use MM Metalizers because they are cheaper and easier to use. They also can provide great results in one thin coat.