I have a few of the HF blocks. They hold up very well. I've done things with them that I'd never do with a DMT or other high end diamond plate. They are quite rough at first (like others), and I could see an uninitiated user thinking they lost their abrasive after they smooth out but mine cut very well. I've had the first one I bought for several years. I do believe that they've changed a little over the years, but I find the new ones to be just as good. The one problem is that the plates aren't flat. I don't mean that they are warped, more likely the process for punching out the swarf holes have distorted the steel and the nickel + diamond isn't enough to overcome the wavy surface. Doesn't seem to make much of an impact on normal sharpening. For a hobbyist sharpener it might be too much. The finish achieved by the "fine" side is not very refined unless you like a lot of tooth. Overall, I think it's a great pickup for the price.
I have mentioned a few times how much I like Martins method for stopping the absorption of mineral oil on my Norton SiC. What I do is rub a thin layer on the surface then hover the stone over the flame on the stove top just for a second. The petroleum jelly turns to liquid seeps into the surface then turns back to its regular state. As HH points out, the oil and jelly mingle just fine. My stone seemed nearly unusable as it would soak (and then bleed) a ridiculous quantity of oil.
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