If a metallic panel is scuffed with 400 would the intercoat fill those scratches enough to apply base over without said scratches showing up?
Even if it filled them, you would still see the scratches as the scratches are a disruption in the metallic surface which affects how the metallic looks. Inter would be no different than regular clearing over a panel with 400 scratches. Don't know if that made sense or not.:)
Metallic the surface has to be perfectly even or else the light reflections will highlight the high and lows of the scratch. Solids you could get away with it as the light reflections are all on a uniform color. Metallic is not uniform in color.
If they are going to do it regardless, I'd tell them to use 2 coats and then wait a good long while before continuing.