I've been using Alclad since it first appeared and have made just about every boo-boo possible before becoming confident of a good result every time. I've used four different Alclads today, though none were a highly polished finish (aluminium, matt aluminium, steel and burnt iron).
For any of the highly polished finishes I would either use Alclad's own base coat or an enamel gloss black. I have used the latter under polished aluminium with good results. As Tim said, be careful not to overdo it, just mist it on.
Here is the truly appalling old Monogram DC 3, resplendent in a gloss black, I think it's probably Humbrol.
And with a polished aluminium finish, knocked back slightly with a varnish.
Like any metal finish, it's 90% in the prep, if you b*gger that up it won't matter what you do with the Alclad. Trust me, I've learnt the hard way