My suggestion is to prime EVERYTHING except the clear parts.
IMHO, 90% of the painting problems like paint lifting, paints not adhreing, incompatability betwen paints and so on can be AVOIDED by priming a Kit and laying down clear coats between paint layers.
I also used to skip the priming phase, till I started on Resin, Vinyl Kits and multimedia kits there I learned the hard way that you should always prime your kits.
Since I started doing this on my plastic models too, the quality of my finishes has improved greatly.