I prime 'everything' because until last March I was building cars and using lacquer paints on my bodies. Lacquer paint will attack unprotected styrene, so I started priming all my car kit parts with lacquer primer to protect the plastic. Now am trying armor, but still prime all parts. Habit I guess!
Someone has suggested that priming will 'hide' details. Well that's true if you put too much primer on! I try to get a even coat on, but just enough to cover the paint. I have flooded an area sometimes and have had to strip the primer off then start over.
The primer should be put on first thing. Let dry for a day, just to be safe. Then place your paint coats over the primer. Enamels, acrylics or lacquer should then go on without any problem. Hope this helps!
Glenn