Let me give your a primer on primer (...). There are three reasons for primer:
- It covers well, in a neutral colour. This is good if you have sanded layers of paints, done repairs, used putty, etc. You want a consistent surface under the paints, because most paints are a little transparent and without primer those inconsistencies may show.
The coverage will also make dents and cracks more visible, so that you can fix them with putty.
- It adheres to the surface and is porous on the top, making paint adhere better. There are different types of primer for different types of surfaces, but there are often "generic" primer that work well in most circumstances, especially for plastic. Use special primer for aluminium, though.
- It sands easily.
If all you are going to do is paint a plastic frame into another colour, then you won't need primer.
No need to cover or expose flaws, If the paint has a solvent in it that also dissolves plastic (I.e. is not oil-based or water-based), then it will adhere well to plastic without primer. No need to sand.
As a general rule, use the same brand and line for primer, paint and clearcoat -- that way you will be sure that they are compatible with each other. That is especially important for paint and clearcoat. Also, don't stray from the instructions on the can.
Clearcoat can add extra protection, and also change the shine. Paints are not always available in matte / satin / gloss, but you can use gloss clearcoat on top of matte paint or vice versa.