Primer the model first is always a good start, forgot that (having a bad day) if doing warships then grey primer is best.
If doing light colours such as white or yellow (few examples) then a white based primer is more suited as the grey can take quite a few more coats to cover with lighter colours,( this tends to lead to loss of detail or thick paint runs to cover if impatient) white primer usually vanished within 2 coats using say yellow or light reds, white top coats usually after one, you can't see but at least 2 is needed, you can normally tell by the finish of the paint if the surface is well covered especially with gloss paints as the patchiness will be matt or satin effect.
finish coats to protect your work if using a matt paint are important ... obvious really, come in a range of finishes matt to match matt paint, but you can protect with satin or gloss over matt paints, i tend to find that a satin finish over either satin or matt colours give the most realistic effect in scale models, matt can be a little to harsh to the eye, gloss way over the top on some models.
Satin is a good all round for many models but the choice is your depending on what your painting and how you want it finished. either way Spray that finish on 1 or 2 coats usually suffices for most models.