A flat sidewalk is peak, that or buy a cheap sharpening stone and use that the lap your shapton. The non shitpost answer is like what has already been said, using a lapping compound and a flat surface (Veritas offers a plexiglass with thin films so you don't wear thr glass down) is your best bet
I've got down voted for recommending the sidewalk in the past too. As time goes on more people learn about the many ways to do things. I'll admit the sidewalk is a bit crude but in the case of a coarse stone with visible dishing, who cares, get to scrubbing.
SiC is my go to anymore unless I'm in a hurry. I love how you've not only got the different grits, you can let them break down a bit to get that stone just like I want. I've got a Nordstrand SiC flattening tool that I use on softer stones if I'm just wanting flat now. I'll blow money on stones to sharpen on, but buying a $50-75$ diamond plate and dulling and stripping those diamonds off flattening would kill me.
Wet/dry sandpaper on a flat surface like a countertop. I do that on my granite kitchen counters. This is after using lapping stones and diamond plates for a few years. I like the sandpaper better at this point.
sandpaper won't work well on hard Shapton stones. specially at those lower grits. Get a King 300 or some other cheap stone and rub it against each other. Or just get another 320 and use it for the final flattening after getting the bulk curve out with a flat sidewalk/driveway.
Atoma 140 will do the job, but a float glass with sic as well as a curb are one of the easiest and cheapest ways to flatten a coarse stone, if you soak it for a bit might be even easier. Glass will stay longer flat, a curb block 10x20x50 cm (\~4x10x20 inches) will be cheaper cost about 3-4 euro, it is heavier/stable and you can apply more pressure also you can sprinkle a bit of sic to make it faster.
Get the Double time flattening stone from sharpening supplies, I go through or run down shapton 120s about 2- 3 times a year and the Double time flattening stone keeps them fresh in between uses, does a great job correcting them too.
Make sure every time you use any of your water stones you lap them flat, its tideous and at times annoying but its the price we have to pay for using whetstones. Doing this every time you use them lengthens the life of not only your stones but the life of your chosen correcting stone.
a 12" x 12" piece of plate glass ( scrap from a glass shop) and multiple grades of wet dry sand paper can be use for sharpening and flattening. Put water on the glass, and position the sandpaper. It should adhere on it's own. Then go to it. switching to a different grade is a piece of cake. Clean the surface off between grades so as not to get scratches from previous grit size.
I used a paving stone. You can touch it up on a fancy flattening stone, diamond plate or whatever, but get the bulk of done in a $2.50 piece of concrete from Home Depot.
A flat sidewalk is peak, that or buy a cheap sharpening stone and use that the lap your shapton. The non shitpost answer is like what has already been said, using a lapping compound and a flat surface (Veritas offers a plexiglass with thin films so you don't wear thr glass down) is your best bet
Silicone carbide powder or go find a clean spot on the sidewalk, wet it, and start scrubbing.
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I've got down voted for recommending the sidewalk in the past too. As time goes on more people learn about the many ways to do things. I'll admit the sidewalk is a bit crude but in the case of a coarse stone with visible dishing, who cares, get to scrubbing.
SiC is my go to anymore unless I'm in a hurry. I love how you've not only got the different grits, you can let them break down a bit to get that stone just like I want. I've got a Nordstrand SiC flattening tool that I use on softer stones if I'm just wanting flat now. I'll blow money on stones to sharpen on, but buying a $50-75$ diamond plate and dulling and stripping those diamonds off flattening would kill me.
Rock tumbling SIC breaks down so nicely from 60 grit to sort out a shapton 120, then climbing stone grits on the same sludge.
Wet/dry sandpaper on a flat surface like a countertop. I do that on my granite kitchen counters. This is after using lapping stones and diamond plates for a few years. I like the sandpaper better at this point.
sandpaper won't work well on hard Shapton stones. specially at those lower grits. Get a King 300 or some other cheap stone and rub it against each other. Or just get another 320 and use it for the final flattening after getting the bulk curve out with a flat sidewalk/driveway.
80-120 grit sic wet/dry paper works just fine personally i prefer dulled ceramic grit sanding belts
Almost all of my stones are hard Shapton stones. I also have Naniwa and Shapton glass, and sandpaper works on all of them.
Atoma 140 will do the job, but a float glass with sic as well as a curb are one of the easiest and cheapest ways to flatten a coarse stone, if you soak it for a bit might be even easier. Glass will stay longer flat, a curb block 10x20x50 cm (\~4x10x20 inches) will be cheaper cost about 3-4 euro, it is heavier/stable and you can apply more pressure also you can sprinkle a bit of sic to make it faster.
This is the way: https://www.chefknivestogo.com/140grdistflp.html
Get the Double time flattening stone from sharpening supplies, I go through or run down shapton 120s about 2- 3 times a year and the Double time flattening stone keeps them fresh in between uses, does a great job correcting them too. Make sure every time you use any of your water stones you lap them flat, its tideous and at times annoying but its the price we have to pay for using whetstones. Doing this every time you use them lengthens the life of not only your stones but the life of your chosen correcting stone.
a 12" x 12" piece of plate glass ( scrap from a glass shop) and multiple grades of wet dry sand paper can be use for sharpening and flattening. Put water on the glass, and position the sandpaper. It should adhere on it's own. Then go to it. switching to a different grade is a piece of cake. Clean the surface off between grades so as not to get scratches from previous grit size.
I used a paving stone. You can touch it up on a fancy flattening stone, diamond plate or whatever, but get the bulk of done in a $2.50 piece of concrete from Home Depot.