You know it! My car has polished primer in some spots. I actually prefer single stage paint for repair as clear coat always peals and once that starts there is no going back.The oem finish on the 1991 was single stage; not base/clear. You can only polish single stage so many times until it you get to the primer. The paint wears down to the metal on the edges first.
Awesome fathering on your part. I would have adored building a car with my father.