# Hand sandpapering in some concavity



## ian (Apr 5, 2020)

Hi all,

I've been tinkering for a while with this Watanabe 180, formerly kurouchi. It's been a fun project: I've ground in some more distal taper, and thinned it a bunch. Here are some current measurements.

Distal Taper (spine width measurements)

3.5 mm above the heel, 1.5 in the middle of the knife, .6 mm 1cm from the tip

Grind in the middle of the knife

.5 mm at 5 mm from edge
1 mm at 10 mm from edge
1.35 mm at 15 mm from edge
around 1.5 mm from 20 mm to the spine

It cuts quite well: goes very easily through hard product, and the food release is not terrible, even though it's a basically a laser now. (I left it pretty fat near the heel where you'd grip it, though, for comfort. Hence the weirdness in the bottom of the choil shot.) However, I still want to improve it if possible. I feel like there could be *slightly* less stiction through wedgy things like cold hard apples, and perhaps food release could be improved slightly. Anyway, I'm just stuck at home looking for things to improve. I could just stop here.

This brings me to the question about concavity. Both Wats that I've tried have had a strip of concavity on the back side of the knife, maybe from 5mm to 10mm from the edge. I preserved this while doing the regrind by wrapping some 120 grit sandpaper around a fat pen cap, and going at the strip for a while, rubbing in the direction of the pen cap. This concavity is very visible when you sharpen, and you can maybe see it a little bit in the choil shot.

*Question: *Would it be worth it to try to put some similar concavity further up the front of the knife, making a sort of S-grind? I don't know if the knife is currently too thin for this, so that the amount of concavity I could fit in wouldn't really affect performance. Should I try to extend the concavity on the back side up a bit, to help with stiction?

Or should I instead try to extend the convexity of the front side further up the knife? Currently, it's more convex within 10mm from the edge, then it flattens out. Given that the angles are so small, I don't feel so comfortable doing this with wobble strokes.... mousepad + sandpaper + strokes perpendicular to the edge or something?

Apologies for the long rambling post, and for the surplus of media to come.





















And here are some videos of me screwing around trying to illustrate food separation and release.


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## M1k3 (Apr 5, 2020)

Try either Jon from JKI Hamaguri technique, Murray Carter's convex sharpening or sandpaper on like a rubber mousepad or similar material.


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## McMan (Apr 5, 2020)

You can't put steel back.
Go big or go home.
Choose one of the above.

I think you've got your mind made up  Go for it! You've already taken the time to remove the KU, might as well keep going and try to standardize some of the concavity. KU hides a multitude of sins. The knife looks very nice, so it's clear you took some time to get the finish/edits right. My vote is keep going. 
This said, I'm not positive a little concavity would add a ton of performance. If not deep and/or tall enough, you could possible see an increase in food sticking to the blade. 

In my experience with S-grinds, they function best when slightly noticeable. So you may get the concavity consistent along the bladeface only to realize it's not quite at the performance you want to see, so time for round 2 to deepen and/or raise the S. I suppose my question would be, Where's the sweet spot for S-grind height? Most makers that do it well (Dalman, Halcyon, Takeda) seem to put it from fairly close to the edge to fairly close to the spine. 

Maybe you could try to just a thin strip of concavity up front to match what's already in back. Then, if you didn't like the results, you could just go elbows deep and raise/deepen the S. Will probably take forever. Probably that's not a bad thing these days.


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## M1k3 (Apr 6, 2020)

I missed you wrote concavity. I personally wouldn't... But to each their own.


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## RDalman (Apr 6, 2020)

Try to find where if you have a small shoulder on the grind 10-20 mm up from the edge. You could probably make a little improvement adding concavity above that. I would look at edm hand finishing stones for this job. Easy to dress to desired concavity, and redress as necessary until you begin getting a channel you're running it it. I don't think the risk is large for this, but don't overdo it and remove too much weight. Also if going for edm stones, don't go for the super hard ones like orange boride edm, but a medium to soft more standard kind, since you're going to be working soft iron.


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## Benuser (Apr 6, 2020)

Just intuitively: I would rather add convexity near the edge, and moving it more to the left.


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## ian (Apr 6, 2020)

RDalman said:


> Try to find where if you have a small shoulder on the grind 10-20 mm up from the edge. You could probably make a little improvement adding concavity above that. I would look at edm hand finishing stones for this job. Easy to dress to desired concavity, and redress as necessary until you begin getting a channel you're running it it. I don't think the risk is large for this, but don't overdo it and remove too much weight. Also if going for edm stones, don't go for the super hard ones like orange boride edm, but a medium to soft more standard kind, since you're going to be working soft iron.



Thanks for the recommendation. I hadn’t thought of using edm stones. Will they be faster than sandpaper? The cladding’s stainless, but it’s not so bad to work, so if so that sounds great. And right above the rounded shoulder is where I was thinking of putting the concavity. Any opinion on how wide to make the strip? 10 mm?



Benuser said:


> Just intuitively: I would rather add convexity near the edge, and moving it more to the left.



Yea, my thought was just that the blade face is pretty flat farther up, so some sort of shape up there will help with food sticking.


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## Carl Kotte (Apr 6, 2020)

Nice project @ian. I’m bored and have a lot of time too, so I’ll be interested in seeing the outcome and then take after (unless you mess things up!). Project knives are fun knives.


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## Carl Kotte (Apr 19, 2020)

Hey @ian, you mentioned in another thread that you sanded your own s-grind. Please show! I’m very excited!


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## Luftmensch (Apr 19, 2020)

ian said:


> I've been tinkering for a while with this Watanabe 180, formerly kurouchi. It's been a fun project: I've ground in some more distal taper, and thinned it a bunch. Here are some current measurements.



Wicked taper!




RDalman said:


> Try to find where if you have a small shoulder on the grind 10-20 mm up from the edge. You could probably make a little improvement adding concavity above that. I would look at edm hand finishing stones for this job. Easy to dress to desired concavity, and redress as necessary until you begin getting a channel you're running it it. I don't think the risk is large for this, but don't overdo it and remove too much weight. Also if going for edm stones, don't go for the super hard ones like orange boride edm, but a medium to soft more standard kind, since you're going to be working soft iron.



Really good advice...

If you don't have access to those and you are a fan of "breaking ****"... the other option is to cut up a stone:






I use this stone for minor shaping and rough finishing work. You can see I have rounded the corners (on the left) on the stone from use. I actually use the side faces and those corners for hollowing (like cleaning up a rusty ura). You can see the small 'finishing stone' at the top right... when that wears too thin, i'll cut another piece off the main stone....




ian said:


> Would it be worth it to try to put some similar concavity further up the front of the knife, making a sort of S-grind?



Hehe... Probably not... I am doing the same thing on a chinese cleaver right now (using that little' finishing stone' above)... not so much because I think it will help that much... but because I thought it would be 'cool' . While it might be a fractionally better cutter (or worse ) and I may be able to shave a few grams off a heavy blade, it is a descent into madness.




McMan said:


> I think you've got your mind made up  Go for it! You've already taken the time to remove the KU, might as well keep going and try to standardize some of the concavity.



I second this☝, with the caveat that you must be happy to take your knife out of rotation and volunteer yourself for many hours of grinding and refinishing... I don't think you'll create a step change in your cutting experience but you will get to feel proud you have personalised your knife even more!


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## ian (Apr 19, 2020)

Carl Kotte said:


> Hey @ian, you mentioned in another thread that you sanded your own s-grind. Please show! I’m very excited!



Here’s the current condition, sanded quickly up to 320. It did improve performance, imo! Little better release, and also improved stiction to some extent. Comments welcome.


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## Carl Kotte (Apr 19, 2020)

ian said:


> Here’s the current condition, sanded quickly up to 320. It did improve performance, imo! Little better release, and also improved stiction to some extent. Comments welcome.
> 
> View attachment 77483
> View attachment 77484


Wow, I’m impressed! How did you do it in the end - sandpaper or the stones mentioned by Dalman? Please post a potato video later!


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## ian (Apr 19, 2020)

Carl Kotte said:


> Wow, I’m impressed! How did you do it in the end - sandpaper or the stones mentioned by Dalman? Please post a potato video later!



Mostly just did it wrapping 120 grit sandpaper around a cork. I bought some EDM stones, but hated them. See this thread.

Here are a couple potato vids with a white potato straight from the fridge.





It's basically a laser now, so you shouldn't expect the little cubes of potatoes in the second vid to just fall off the blade. But they brush off super easily. The very thin slice in the first vid was more representative than not of the rest of the slices. There were a couple thicker ones, as there usually are, but I was trying to cut them super thin.

The real pleasure in using this knife, though, is that throughout both videos I could barely feel the potato while cutting through it. Food separation to the max!

Edit: I learned from some other users about the drama of having the cutting board move toward the camera when chopping, so I tried my best to implement this technique in the second vid.


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## Carl Kotte (Apr 19, 2020)

@ian Love the drama and your trousers. Now go knock off some handles!!! (I’ve managed to use a cutting board to remove a handle with when I had no plank to hammer on).


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## ian (Apr 19, 2020)

Carl Kotte said:


> @ian Love the drama and your trousers. Now go knock off some handles!!! (I’ve managed to use a cutting board to remove a handle with when I had no plank to hammer on).



I thought about that, actually! But didn’t want to f up my boards. I may ask around the block and see if anyone has a 2x4.

Does anyone wear anything but pajama pants nowadays?


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## McMan (Apr 19, 2020)

ian said:


> Here’s the current condition, sanded quickly up to 320. It did improve performance, imo! Little better release, and also improved stiction to some extent. Comments welcome.
> 
> View attachment 77483
> View attachment 77484


That came out nicely! 
Phase 2--Kasumi time?


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## Carl Kotte (Apr 19, 2020)

McMan said:


> That came out nicely!
> Phase 2--Kasumi time?


Phase 3: tattoo/etch your own damascus pattern on Victorinox parers?


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## Carl Kotte (Apr 19, 2020)

I’d say this is a success story. Thank you Ian! Now I must follow you in your footsteps. I just have to decide which knife to grind...


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## Luftmensch (Apr 19, 2020)

Nice one @ian! Success! Congratulations!

How deep did you go? If you put a ruler on the hollow, how much of a dip did you create?


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## ian (Apr 19, 2020)

Luftmensch said:


> Nice one @ian! Success! Congratulations!
> 
> How deep did you go? If you put a ruler on the hollow, how much of a dip did you create?



It’s subtle. I could probably go deeper if I wanted, but got a little bored, and I’m not bothered by the performance anymore. Don’t know exact numbers, but here’s a pic.


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## ian (Apr 19, 2020)

That said, if someone out there says “that’s not really deep enough, you should go deeper” I’ll probably go back to it again.🗜




McMan said:


> That came out nicely!
> Phase 2--Kasumi time?



Might hold off for a bit and make sure I continue to like how it cuts before polishing further. I took it to 600 before and then decided I needed to make an S grind, so wasted a bunch of time.


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## Carl Kotte (Apr 19, 2020)

I’m so bored I’m considering learning how to sharpen my bread saw - using only a toothbrush. I’ll post results in this thread.
But first I have an old Masahiro suji with a lot of pitting that could use an s-grind. Try to stop me!


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## Luftmensch (Apr 19, 2020)

ian said:


> Might hold off for a bit and make sure I continue to like how it cuts before polishing further.



That is wise... The S-grinds I have seen are subtle - they aren't too deep. You need a ruler to see them.

See what the makers say... they better experience to draw from!


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## Luftmensch (Apr 19, 2020)

Carl Kotte said:


> Try to stop me!



Im not gonna...


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## M1k3 (Apr 19, 2020)

Carl Kotte said:


> I’m so bored I’m considering learning how to sharpen my bread saw - using only a toothbrush. I’ll post results in this thread.
> But first I have an old Masahiro suji with a lot of pitting that could use an s-grind. Try to stop me!



I'll bite. Magic bristles or wrapping sandpaper around the handle?


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## Carl Kotte (Apr 19, 2020)

M1k3 said:


> I'll bite. Magic bristles or wrapping sandpaper around the handle?


What Would you suggest? Bristles sounds good


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## McMan (Apr 19, 2020)

Sharpen the toothbrush and you'll have a whole different kind of tool...


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## ian (Apr 19, 2020)

Luftmensch said:


> See what the makers say... they better experience to draw from!



From above:



RDalman said:


> don't overdo it and remove too much weight



Sage words. For some reason, though, I thought I remembered you saying that the performance level is a direct function of the depth, seeming to imply that a little concavity might not do much.

Also, regarding the EDM stones, I may have just gone and bought exactly the ones that @RDalman warned me against.  For some reason, I thought the most commonly used ones would be the right ones, but those were the orange ones. Sometimes I’m not so smart.


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## M1k3 (Apr 19, 2020)

Carl Kotte said:


> What Would you suggest? Bristles sounds good



Depends on how big the scallops are and how round the handle is. Maybe just sharpen the toothbrush and buy a new breadknife? And turn the old breadknife into a knife?


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## Luftmensch (Apr 19, 2020)

ian said:


> For some reason, though, I thought I remembered you saying that the performance level is a direct function of the depth



You might be able to summon @Kippington . I think there was a discussion on the step size in the hook-grind thread??


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## Kippington (Apr 20, 2020)

Luftmensch said:


> You might be able to summon @Kippington . I think there was a discussion on the step size in the hook-grind thread??


That's a little bit confusing. Lemme clear it up.
The step size I was talking about is measured from the edge towards the spine. The smaller this measurement, the better the food release. Make the step height really small and you get fantastic food release at the cost of the lifespan of the knife (in regards to food release).


The depth doesn't matter as much, depending on what you're cutting. Thin and flexible slices of food can conform to a shallow step, making it redundant... and that's really as complicated as it gets.


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## Kippington (Apr 20, 2020)

Luftmensch said:


> That is wise... The S-grinds I have seen are subtle - they aren't too deep. You need a ruler to see them.


I really like using reflections to see the grind.
I can see what Ian has done to the knife by looking at the shadows and reflections on the knife in the left of this thumbnail picture (without playing the video):

You can pick up on some nice subtleties using reflections, things that a choil shot won't show about the grind.
Take a look at the reflection on the following knife vs the choil shot. You can clearly see a slight concavity running through the middle of the blade in the first picture, but it's very difficult to pick up in the second one.


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## M1k3 (Apr 20, 2020)

I picked it up in both pictures


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## Kippington (Apr 20, 2020)

M1k3 said:


> I picked it up in both pictures


Then you, sir, have better eyes than me!


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## RDalman (Apr 20, 2020)

Near edge geometry and overall handling like weight and profile, makes much bigger impacts on performance than prescence of concavity. Like the last kip posted now, a strong convex near edge separates enough anyway so concavity would matter very rarely.


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## Luftmensch (Apr 20, 2020)

Kippington said:


> Then you, sir, have better eyes than me!



Better than my eyes as well! I can't see it...


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## Luftmensch (Apr 20, 2020)

Kippington said:


> The step size I was talking about is measured from the edge towards the spine. The smaller this measurement, the better the food release. Make the step height really small and you get fantastic food release at the cost of the lifespan of the knife (in regards to food release).



Ahhh! gotcha... 



Kippington said:


> The depth doesn't matter as much, depending on what you're cutting. Thin and flexible slices of food can conform to a shallow step, making it redundant... and that's really as complicated as it gets.



Yeah - thats my intuition. Thin slices and dicing are flexible materials. That said... A larger depth may help for large sticky things like potatoes? Does this hold water?


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## Kippington (Apr 20, 2020)

Luftmensch said:


> A larger depth may help for large sticky things like potatoes? Does this hold water?


It depends on the rigidity of the cut item.

If we use potatoes as an example - if you take an uncooked potato and cut it into thick pieces, the food stays hard and has little chance of flexing to conform to any extended grind geometry. In this case, the step depth is negligible, and the food release will depend mostly on the grind geometry just behind the edge.

If you switch it up and try to cut a wafer thin slice of the same raw potato, now the cut item can flex and bend. As it slides up the side of the knife, it might follow the contours of a soft shoulder and continue to stick inside a concave section. In this case, a hard shoulder, or step, might help discourage this behavior.

This specificly only applies to the angle and curve of a shoulder, and ignores other important aspects of cutting in general (like speed, relative motion, surface finish and moisture content), so it should be taken with a grain of salt.


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## Luftmensch (Apr 20, 2020)

Kippington said:


> if you take an uncooked potato and cut it into thick pieces, the food stays hard and has little chance of flexing to conform to any extended grind geometry. In this case, the step depth is negligible, and the food release will depend mostly on the grind geometry just behind the edge.



In this case, my instinct is that within reasonable limits, a bigger 'gutter' might help break the surface adhesion between the blade face and the potato. That starchy water goo might not be able to seal... I am not sure what bigger is though? Something in-between imperceptible and the Challenger Deep 

And then again... maybe it is moot because you have enough adhesion further up the blade...


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## Carl Kotte (Apr 20, 2020)

M1k3 said:


> Depends on how big the scallops are and how round the handle is. Maybe just sharpen the toothbrush and buy a new breadknife? And turn the old breadknife into a knife?



I just brushed my teeth with the bread knife. That was a bad idea and I really don’t recommend it to anyone.


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## panda (Apr 20, 2020)

ian said:


> That said, if someone out there says “that’s not really deep enough, you should go deeper” I’ll probably go back to it again.🗜
> 
> 
> 
> ...


That's what she said


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## panda (Apr 20, 2020)

I first cringed at this but then realized it was for a nakiri in which case do what ever the [email protected] you feel like, lol.

That being said, the beauty of wat grind is the tall subtle convexity which leads it to cut so smooth yet constantly push food away.


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## ian (Apr 20, 2020)

panda said:


> I first cringed at this but then realized it was for a nakiri in which case do what ever the [email protected] you feel like, lol.
> 
> That being said, the beauty of wat grind is the tall subtle convexity which leads it to cut so smooth yet constantly push food away.



It’s a gyuto, not a nakiri. Do you care about any knife < 240mm, though? Also, it was originally KU, and taking off the KU changed the original grind anyway. Whatever, though. There are certainly cheaper knives to use as a project, but I actually like using this one, whereas my other cheaper project knife won’t see much use when I’m done, I imagine. You thinkI should send Watanabe an email apologizing for destroying his knife? 

—-

@Kippington, I feel like the concavity may also help just because now there’s more curvature in the blade face further up the knife, so flat things (eg little cubes) have a harder time sticking to it. Maybe they still stick, but they’re easier to brush off. All this is subtle, though, so I may be imagining some of it. And also, I feel like there might be slightly less stiction going through something like an apple. With a really thin knife, this can be a problem if the apple contacts the knife in a large area, but now there’s a little less area to contact with. Maybe this is all my imagination too, though.

Do you think I’m just believing in ’s here?


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## panda (Apr 20, 2020)

Ahhh I had just woken up, excuse my lack of reading comprehension I am not a morning person what so ever.

Haha no don't email him, I like that you're going through with major changes in the geometry. I modify every knife I own..


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## Kippington (Apr 20, 2020)

ian said:


> @Kippington, I feel like the concavity may also help just because now there’s more curvature in the blade face further up the knife, so flat things (eg little cubes) have a harder time sticking to it. Maybe they still stick, but they’re easier to brush off. All this is subtle, though, so I may be imagining some of it. And also, I feel like there might be slightly less stiction going through something like an apple. With a really thin knife, this can be a problem if the apple contacts the knife in a large area, but now there’s a little less area to contact with. Maybe this is all my imagination too, though.
> 
> Do you think I’m just believing in ’s here?


Sounds about right to me.

As @Luftmensch said, you can think of stacking fine dice as a sort of continuous flexible slice in terms of food release. A sharper shoulder somewhere up the grind will encourage good food release.

For stiction: Yes, less surface area in contact with the knife will help reduce drag.

But I think I'm just helping to confirm what you already know...


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## ian (Apr 20, 2020)

Kippington said:


> Sounds about right to me.
> As @Luftmensch said, you can think of stacking fine dice as a sort of continuous flexible slice in terms of food release. A sharper shoulder somewhere up the grind will encourage good food release.
> For stiction: Yes, less surface area in contact with the knife will help reduce drag.
> 
> But I think I'm just helping to confirm what you already know...



I mean, the theory of all this makes perfect sense to me, but some things that make sense in theory end up having a completely negligible effect on performance. That is, I know “doing this will make it better”, but “better” might not be noticeable. I guess I was just looking for validation, because I was afraid for a moment that I’d fallen into the trap of “I just worked on this knife for 15 hrs, so now it must cut better!”


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## Luftmensch (Apr 20, 2020)

ian said:


> “I just worked on this knife for 15 hrs, so now it must cut better!”



I don't believe I have ever made that mistake... ever.... .... like 17 times.... 

Hehe... I am glad you are feeling good about it. What sort of finish/polish are you thinking about?


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## Luftmensch (Apr 20, 2020)

Luftmensch said:


> like 17 times....



And probably on the same knife


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## ian (Apr 20, 2020)

Luftmensch said:


> I don't believe I have ever made that mistake... ever.... .... like 17 times....
> 
> Hehe... I am glad you are feeling good about it. What sort of finish/polish are you thinking about?



Probably just sandpaper up to 800 or 1000. Then I may try the fingerstones I got from @nutmeg, although I know the finish they leave won’t really match the sandpaper, so maybe that’s a losing proposition. May also just try King 800 mud on a felt pad or something.


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## Carl Kotte (Apr 23, 2020)

A subtle concavity:


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## soigne_west (Apr 23, 2020)

I wonder if one of these could help add some concavity to a knife


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## tostadas (Feb 19, 2021)

Bringing this thread back to life. I'm curious to try something like this. @ian any long term thoughts on this project?


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## ian (Feb 19, 2021)

Hmm, it was fun! It did make the knife lighter and whippier, so just make sure you know that you’ll affect the feel and balance of the knife when you do it. The knife did perform pretty well after... crazy food separation from all the thinning, but not terrible food release. I sold the knife a long while ago, though.

My biggest regret is using the nonword “sandpapering” in the thread title. Not sure why I wrote that.


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## tostadas (Feb 19, 2021)

Hmm.. If I still had my hefty mazaki, that would interesting to try with


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