Clay then AIO IMO.
I dont disagree with your comment Mark, but if you dont clay on a car that hasnt not had it for a few years and has seen 5K+ miles then your can end up marring the paintwork with the dirt/tar/bonded junk, esp. if you apply the cleaner with a machine as it gets trapped between the pad and the paintwork.
But if you hit major bonded junk then your going to end up transfering it to the pad or applicator.
If you want best durability from a sealant such as Zaino, which bonds rather then a wax which lays on the surface then you really need a chemically clean paintwork surface.
Also, sealing/coating bonded fall out in to the car can increase damage done by the fall out.
Geoff