Sounds like your doing a thorough paint prep there Brett. I'm wondering though if the Waikato water may be leaving a residue ?
I do all the steps your doing but after I've washed the body and let it dry I always give it a wipe over with a rag and Fuelite.
I do this even between coats if I have sanded or handled the body in any way.
Fuelite is just a fast drying no residue degreaser widely available locally at the likes of mitre10.
The likes of Repco/ Supercheap Auto could sell you a prep wash, wax & grease remover that would do the same thing but cost a lot more.