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I was ONLY trying to learn.........
Regards
Jay
Don't sweat it, Jay.
Try it both ways and see what works best for you and your preferred painting materials and techniques.
As for myself, I've been building armor models non-stop for almost 50 years, and I never regularly prime vehicle models.
I do always wash my builds before I start painting - ordinary liquid dish detergent, warm water and a soft water-color paint brush. This removes sanding dust, swarf from sawing and drilling, and other building "schmutz" along with oils (I use a lot of 4x0 steel wool) and mold release agents, etc.
I only use acrylics (mostly Tamiya, but also other brands) and have absolutely no problems with paint adhesion even for models that are decades old. Zero, zip, nada - no problems at all.
(For example, I have a Italeri Pz I with "On the Mark Modeling" PE, built ca. 1988, an IPMS Nationals first place winner, that hasn't lost so much as a flake of paint in all these years...)
I am very careful in how I handle and how much I handle my models as I'm painting them. I always wash my hands before I sit down to do any painting and deliberately try not to handle things more than necessary. (Greasy pizza finger prints, dirt, etc. on your hands will leave blemishes in your finish. Even if you think your hands are clean, they probably aren't clean enough to handle a painted model...)
Having said all this, though, I always prime my figure models with Tamiya flat white before hand brushing the acrylic undercoats, but that's entirely because I want to ensure that I get the exact colors that I want on these undercoats.
If you find that you're rubbing the paint off your models as you finish them, then maybe you need to prime or handle the models less. If you find that the paint stays where you put it, then maybe you don't need to prime. If you find that you need so much paint to get good color coverage that you're hiding the details, then maybe you need to prime so that you don't need so many extra color coats...
I'd say, when or when not to prime depends more on you than on what anyone here says.
HTH,