Alclad only needs the gloss black primer under the highly polished finishes. I haven't looked recently, but they did make this clear on their web site. I us a normal grey primer under most typical natural metal finishes, in my case usually versions of aluminium.
Rather than buying several slightly different shades of a relatively pricey product you can tint Alclad with a drop or two of gloss black (or white) to give some variation, as in the panels on the bottom of this Ta 152. You can then spend the money saved on a nice bottle of wine, or whatever takes your fancy
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There did used to be problems with Alclad's own gloss black primer but I understand that these are now resolved. I used good old Humbrol gloss black on my model above but there are caveats. The primer, whichever you use, must be completely dry and cured before applying the Alclad. I left mine about a week. Alclad is lacquer based and the solvent is quite 'hot'. It will react with some primers to create a nasty looking crazing or orange peel effect. For the same reason you must mist the Alclad on in light coats. If you actually wet the primer with too much Alclad you will be in trouble. To this I can testify from first hand experience
Natural metal finishes are not difficult in my opinion. There are several options out there nowadays and they all seem to give results we would have died for twenty years ago! The trick, just like painting your living room, is to get the preparation right. Natural metal finishes do tend to show up even the slightest flaw in the plastic or your techniques.
Cheers
Steve