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I myself use an automotive paint, decant it and put it straight trough the airbrush. I’ve used Alclads Gun Metal in the past once and although it was a great paint in terms of spraying and looks, it wasn’t worth it for me. In any case, you can spray all types of paint on the ABS, as long as you use a primer, even if it is lacquer. The primer I use is by Alclad and this is a lacquer based primer. Never had any issues with the plastic getting brittle or some other nasty side effect.
Personally I like a gloss better, especially when it has to represent a metal frame (gun metal). On the other hand, I can imagine that a constructor doesn’t want a multi million prototype (Gundams) or production models to rust and therefor give it a “special” coating. Take a Caterpillar engine for example, all painted yellow for maximum protection.
So either way, in my opinion it’ll look cool.
Happy building!