Alclad's "high-shine" finishes are the ones that require gloss black underneath. So Chrome, Polished Aluminum, Airframe Aluminum, Stainless Steel, Gold and Brass.
The regular finishes (Aluminum, Duraluminum, Steel, etc) just need grey primer to protect the plastic, and it's recommended to polish them smooth with 1000+ grit sandpaper.
All per Alclad's site.