My point is that primer is not a top coat, it is part of a system. You can prime it and leave it as is, plenty of people did that to their cars in the 1970's, some still do...but the product is not designed to be a durable top coat and as such is not impervious to the elements. Moisture will penetrate it. There used to be some filling primers that could be applied and would be weatherproof provided they were not sanded, the last one I used was a Sikkens product. Don't recall the name but it was years ago and designed to apply with spray equipment.
Basically primers are designed to prepare the base surface to receive a finish, promote adhesion between substrate and finish and prevent corrosion (depends on primer and surface being applied to). Some are thicker "high build" primers and used for the filling of small imperfections in the substrate.
For clean bare metal, I would use an etching or epoxy primer. Generally speaking, rattle can products will not be as robust as shop-applied multi-part systems because they are not catalyzed...they dry through evaporation, not chemical interaction, and therefore are not as durable. That said, we work with them because we don't have pro spray equipment and paint booths.
There are some 2 part catalyzed rattle can products available, SprayMax is one that comes to mind...
https://www.amazon.com/dp/B07CXPQ814/?tag=wranglerorg-20
https://www.amazon.com/dp/B00W2D806Y/?tag=wranglerorg-20
I've used the clear before with excellent results, I'll be trying the epoxy primer out shortly for a spot repair on my LJ.
These will be more robust because they harden and cure through chemical reaction, therefore producing a more durable substrate and finish. Bear in mind that all of this depends on the condition of the surface you are spraying. Bad surface prep will doom even the best products out there. It's 99% of the job.
I can't speak to one shot primer/paints, rusty metal primers or "rust converters", I don't use them.
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