@Arnout thank you for taking the time to share all this detail with me, it is very informative. I do not have a slurry stone and I used a DMT to make the slurry. Before going to the J Nat I used synthetics to set the bevel and the worked it up to 8k on Naniwa SS stones. I am not even sure if the stone I have is good, it was rated as 4+ for hardness by the seller. The Zulu is pretty easy to use, it came with slurry stone but I use a DMT to build the slurry. Also I only use it as a finisher, it takes a lot of trips up and down that stone to get things nice. It does produce a silky smooth edge but just not as sharp as I like.
4+ should be hard enough, and plenty people use a dmt with good result,.. Did you try jnat slurry on the 20k already?
The 20k becomes your carrier, make slurry with the jnat. After the 8k, renew the slurry at least 3 to 5 times. At the end you can choose to finish on slurry, or on water or inbetween. First time slurry use a bit of pressure, end without pressure. Find something that works for you
I hope you 'll enjoy it! A tip, Rinse the stone, the knive and your hands each time you renew the slurry, it is hard to believe what kind of scratches loose 20k particles or housedust Make!