The word "primer" can mean a number of things. I prefer the term "undercoat" for what you put under Alclad. I prime plastic first with a primer that aids paint adhesion. Then I apply a gloss black undercoat, then the Alclad. Sometimes I will add some other dark gloss colors over the black, in a pattern, corresponding to panels, to get some variation to the Alclad appearance.
The undercoat must be flawless, because you put the Alclad down very thin. So I often find it takes several coats of the undercoat, sanding and fixing flaws as necessary.
Also, I do the alclad last, to avoid having to mask over it. So if the model has other colors than just the alclad, I paint those colors first (over the primer) and then mask those colors and do the black and Alcad.