Umax,
I almost always prime before I topcoat.
The type of paint I use depends on the subject and the material I'm painting onto.............
I like to use automotive acrylics for cars because It can be polished to within an inch of it's life for that "glassy, wet" look.
I always use gray primer for black topcoat, and white primer for other colors - especially gloss white, yellow, reds or blues.
For armour, I often use a primer color that fairly closely matches the topcoat color - and sometimes I just paint the bare plastic with the topcoat as armour sees hard use and doesn't need to be "pretty" to be effective.
For aircraft, I do a lot of airliners, 1/144 scale, and as they never really have that much in the way of detail, I usually make them into the "travel agent" style model with blacked out windows and a glossy paint scheme - then I usually use a flat white primer, then hit it with Future for a gloss, then decal, then Future again.
There are about a thousand different techniques that people will use that differ from mine, but these ways work for me.
Cheers,
LeeTree