On an MDOT-type highway job, when is a prime coat used instead of a tack coat?

1. On a clean, existing asphalt or concrete pavement immediately before an overlay
2. On an untreated aggregate base or granular surface to bind fines and promote bond of the first asphalt course
3. On the finished HMA surface as a substitute for a fog seal after traffic is on the road
4. Only on airport runways, never on public highways