Car body surface prep - all or nothing?

My old car needs a paint job. Bought it used so don’t know the history.

The top coat of paint is of poor quality and is not well adhered to the multiple layers of paint beneath it. It is flaking off in some places. It has to go.

Beneath that, is another layer of paint from previous body work. This layer is better adhered and better quality but I figure it should come off as well.

The third layer of paint is the original factory paint. I wanted to keep this intact and simply scuff the surface of it.

Unfortunately, at least on the first fender I started on (it appears to have been repaired multiple times), the original paint coat was not completely intact. When sanding, I ended up down to bare metal in some parts while in others the factory coat was there.

Question:

Since it’s down to the metal in some places, must I know take it down to the metal throughout the whole piece?

I was under the impression that you need to abrade the metal for primer to adhere properly and I don’t see how you can do this on just a section of bare metal adjacent to non-bare metal.