As the subject of coarse stones is often a highly debated topic, I take advantage of my recent acquisition of JNS 220 and JNS 400 to share my experience in this thread.
The reasons that motivated my choice are multiple: after owning and testing numerous stones for repairs, thinning, and more recently regrinding and polishing, I always encountered the same dilemmas: the fast and softer stones needed to be flattened every 2 minutes, and the harder, more precise ones were also slower. Of course, they're diamond/Atoma type stones which seem like a good compromise, but lose efficiency with wear.
In short, I hesitated before investing in these two stones, but the description was promising and I had already had good experiences with JNS, so I went for it.
Both stones are of good size (approx.1kg, 200X85X30 mm), they come already sealed and relatively flat. A quick pass on a diamond stone was enough to get them ready for action.
For a thinning operation, I started with the 220 grit stone. After a few passes, a slurry develops, but quite reasonably.
Very quickly, I am convinced by the very pleasant feedback which reminds me of that of a Cerax 1K (soaker), quite chalky, slightly coarse, but without any unpleasant gritty/sandy sensation. You can feel that the stone is doing its job. Here is the scratch pattern:
The logical next step is to use the JNS stone to refine the scratch pattern... Again, the sensations are good and similar to the 220, but the grit is of course finer and the feeling is logically firmer without being hard. Here is the scratch pattern:
So, the verdict:
The positives: the feedback, the size of the stone (which partly compensates for its higher price compared to competitors), the fast cutting, the slurry management.
The negatives: one must avoid applying strong pressure on the 220, and preferably use the entire surface of the stone as much as possible, as the risk of dish remains significant, especially if, like me (a beginner in polishing), one stays too long on the left or right side of the stone to maintain consistency as much as possible.
In any case, they're worth a try if you're looking for an alternative to the popular shapton and naniwa models.
Hope it helps!
