1. Most primers have an envelope of around 10 to 48 hours between their application and spraying the top coat. Exceeding that requires scuffing. I would assume applying the top coat after several years is not practically possible. You will need to remove of heavily scuff and apply a new layer of primer/sealer.
2. Again, most primers are not UV resistant, hence they may age when exposed to sun light.
3. Certain brand top coats require specific primers or in other words the top coat manufacturer highly recommend what to use.
Applying primer to the exterior as preparation to painting years ahead of time is practically does not work imho.