Can I get some advice on primer and sandpaper?

[BabyBenz84]

Would you say 80 grit on a DA is acceptable for any areas that you need to go down to the metal? Is it adequate for removing rust?

Is etch primer really necessary if you are shooting primer immediately after exposing the metal? Is it highly advised in areas that previously had rust?

I am thinking of shooting a high build primer. Am I going to want to dry sand, wet sand, or both? Can I dry sand the first couple of coats and then wet sand the final coat? And again, what grits would you guys recommend for this task? Can I use the DA for the dry sanding the primer or do I should I use a sanding block?