Pepe-
I agree that a uniform primer coat is a must. If you have for example gray auto primer showing through a thin white sealer coat, you'll be amazed when you lay down (for example) yellow airbrush paint, how mottled the surface will look. This is what I do:
Starting with the completed model:
-Fill anything that wants to be filled with CWF or even lightweight drywall material.
-A double coat of auto filler primer (I use Rustoleum gray Filler Primer from the auto section of Walmart). Double coat means I spray a medium heavy coat, let it tack up a few minutes, and spray again.
-When thoroughly dry I sand it down so most of the filler is removed, but not sanded enough to raise fuzzies from the cardboard tube...
-I then shoot a sealer coat. These days it's (Createx) Auto Air Sealer White or Sealer Dark. This is a reasonably opaque white or black airbrush paint that sticks very well, and has a matte-ish surface that is perfect as a base for the color coats. I use a mini spray gun ($12 from Harbor Freight) and blast this stuff on. Takes just a minute or two. Longer to clean out the gun than shoot the paint.
-If the model is going to have white parts, I use an opaque white airbrush paint (Auto Air Opaque White, or Wicked Opaque White are my faves) and spray the whole model.
-Then I mask and shoot the color coats. I use Tamiya masking tape and use a thin coat of transparent base applied with a small brush at the edges of the tape, to get sharp lines.
As for clearing, yes this is required. I usually clear with Valspar Clear Lacquer but I have also had pretty good results with just Future (whatever it is called now... Pledge with Future Shine Tile something or other). Just spray it on in light coats, or carefully brush it... it can cause the airbrush paint to bleed a bit in some conditions. It won't be a hard surface, but it stands up to my launching OK. I've also used Liquitex high gloss clear varnish (it's water based), and it works OK but tends to dry with runs or the occasional bubble.
Good luck and let us know how it turns out.
Marc