I can't stress this enough; When picking out any primer you work backwards from the kind of paint you are going to use. You pick out the brand of paint and then you don't get a lot of choices for primer. The paint store guy will show you which primers are compatible.
Hey Jim, we agree on something! You start off learning the product sheets (P-sheets) on the paint product line you intend to use after visiting a local vendor who will service you start to finish with the necessary products.
You chose a paint system within your budget and goals. Then you follow the system start to finish. The most important section on a product sheet is what the product is compatible over top of and what products are intended to go over top. On restoration of a 40 year old car, bare steel is where you need to start.
The primer "open" window is pretty important. I see cars at shows that were top coated after the primer was fully locked up. Those are the ones where the paint lifts in a big chunk from minor impacts that should leave at most a scratch.
Most primer products have a short "open window" somewhere between one day and one week before top coats are applied. Few restorations proceed at that wonderful progress rate unless you have the cast of "Overhauling" working on your ride. If you miss the window for top coating, you need to scuff the primer and apply more of the same primer before applying the top coats. That means if you store a project a under primer and its a while before you complete your repairs, you need to scuff the job, and apply another coat of that primer product before you continue to the next steps. This is because the bond between primers and top coats is chemical.
Since many paint products say "can be applied over OEM sanded finish" that hints at deviations from good practice that "should" work. This advice on the product sheet means on your 1998 Honda, you can freshen up the paint without stripping to bare metal. This has no relevance to forty year old car restoration. On a forty year old car, you have to assume the OEM finish is not a satisfactory foundation for new paint. On a forty year old OEM finish, you would ignore the advice "over sanded OEM finish" and go back to bare steel so your job will last. Exception might be if there is a more recent refinish job that involved modern products applied over clean prepared steel.
You start off learning the product sheets (P-sheets) on the paint product line you intend to use after visiting a local vendor who will service you start to finish with the necessary products. 
