Diamond Paste - How to?

B

Ben Thomas

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Jun 21, 2020
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Hi all. I'm trying to polish up a Magnacut gyuto by hand. I've gotten it to about 400 grit scratches using DTM stones and I have purchased diamond paste in 500, 800, and 1200 grit to take it higher. Any tips or tricks to using the paste for hand sanding? Assume I know nothing and you will be pretty close to the truth.

I played around with the 500 grit using some 800 grit sand paper and my usual backer and it seems pretty effective, but I know this can't be the best approach.

Thanks
 
What diamond paste did you get?? I use some stuff on leather strops for sharpening, but haven't tried them for polishing? Is your paste designed for a buffing wheel, or on the leather strop?

I got some Sic Powders (220, 500, 1200) that I used on a Magnacut gyuto combined with Sic sandpaper and oil on the leather backer that worked decently. EDM stones also work well on magnacut and make the hand finishing at the higher grits easier. All of the abrasives can leave deep scratches in the direction you are using it in, so make sure to change directions and get the previous scratches out!
 
I grabbed a pack of diamond paste syringes for a cheap price to practice with on laps but tried them with blades as well. I use 5 and 2.5 on leather for stropping. For hand sanding/polishing, I use paper to 400, then stones from 4-6 and 800 and depending on the blade, I use 14-10 micron paste with a piece of stainless bar and a touch of either mineral oil or cutting oil. I put a bit (slight bit) of paste and oil on the blade, smear it around with my fingers and then polish with the steel as if I were hand sanding. I check a spot every now and then in different areas and dont wipe off until I think its ready. when using the paste, I only go in one direction as the slurry can leave weird marks and at this high polish, you can very quickly see where you need work.
 
That is for power polishing on gemstones. What you want is graded SC [powders/ Make a thin paste with oil and use with plenty of elbow grease and a hard backed leather pad. Use the pad for only one specific grit. Wash everything between grit changes.
 
I did a magnacut gyuto S grind with the Sic powders 500 grit and it gave a nice finish, but was more matte than shiny. I used 500 grit SiC paper, 500 grit Sic powder mixed with mineral oil and use a piece of leather as the backer. It gets messy and the oily grit gets EVERYWHERE! I haven't tried re polishing with the 1200 grit yet; I cleaned it back up on the belt sander (made the hollow a little deeper) and left it at the scotchbrite finish.
 
The 400/500 grit is to get a matte base. From there you go up the grits, just like using papers, until you get the finish you want - 800/1000 ... 2000/2500 .... 4000/6000 ... etc. At 8000 you probably could switch to diamond paste, but I would not suggest it.
 
Thanks for the feedback. It's there a particular source/brand for the SC powder?

Is the SC over diamond mostly a cost thing or is there a performance difference (i.e. should I wait for the SC powder or go ahead and use the paste I have in hand)?

Does the cutting wear out quickly that I will need to keep reapplying grit?
 
Black Silicon Carbide grit and diamond paste work more or less the same. SC will break down somewhat (becoming a finer grit), but the difference isn't significant. Cost is hugely different.
Most any accurately graded SC powder works. Graded powders are more carefully screened and often have a specs sheet (like good steel) showing the exact particle size and how much variation is allowed. They are also more expensive.
I have used the cheaper grades for tumbling rocks with no bad results. Most problems come from contaminating them in your shop by getting stray grit all over the bench and hands.

You can buy high quality SC as a paste, but it is cheaper and easier to make your own with some oil and powder. Some folks use silicone grease as the carrying agent because it makes a better paste. I use silicone oil. I don't think it really maters what oil or grease you use.
 
I just got a new 2x72 leather power strop belt and am loading it with diamond paste... I bought a selection of 12 grits and will report back. I made about 5000 knives with my last one loaded with pink compound.
 
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