First of all, match all the paint you're going to use, ie: enamel/enamel, lacquer/lacquer, acrylic/acrylic.
Generally, if I want a mirror type finish, or a rubbed metal type finish, I'll proceed along the following steps:
(1) Wash the plastic with water and a small amount of dish detergent to eliminate any grease... (bare fingers should NOT touch the car body from this point on).... you can use a primer if you want to, but if you've cleaned it good there won't be much need for that. Spray the color paint in several light coats, so no running or orange peel occurs, then place under a box for 2 days.
(2) Apply Testors Gloss-Coat to the assembled body and place a box over it to keep dust/fiber particles from settling on it, don't touch for 4 days.
(3) Apply decals and let stand for 2 days.
(4) Apply another good coat of Gloss-Coat and place under the box for another 4 days.
(5) Lightly apply some automotive rubbing compound with a cotton ball to reduce the super glossy finish.
You can achieve a beautiful finish with raddle cans or an airbrush, but using different mediums of paint is terribly unforgiving. Enamel Primer, enamel color, and enamel clear coat.... the same with Lacquer or acrylic.
Hope this helps...
Frank