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Get a cheap steel cookie sheet from a dollar store and find a smooth piece of cement floor. Sprinkle on a teaspoon of grit and squirt of water and use you body weight, spin the stone frequently, add...

Correction, a full pound of 60 grit Silicone Carbide is $4.40. Way more that you will need.

Cheap 60 grit Wet & Dry is $20-30 per 50 pack. Good 3m paper is $110. Good sandpaper is worth the extra price.
As said, you will burn through a pack, especially as worn as you stone appears to be,...

Yup, looks like a well-used Washita, with a swayback.

It will take some work to flatten with 60 grit. But doable and worth the effort.

Nice stone what are the dimensions?

Oven cleaner will remove the bulk of the surface crud, then soak for a day in Simple Green or any good degreaser. That will tell you if it needs more.

Most of these stones were used for years with...

Hard to say from the pic. Remove it from the box and spray it with oven cleaner to remove the bulk of the crud, scrub with a stiff brush.

A long soak in Simple Green or any good degreaser 50...