Is it reasonably flat and or smooth?

Smooth is more important than flat. If it is smooth, bevel the edges with a Silicon Carbide stone and either put some oil or soapy water & Smith’s Honing Oil on it and hone an edge.

Do not lap it unless it is not smooth or way out of flat. Years of use have made it smooth and if you lap it you will have to burnish it with many many laps to get back to where it is now.

You can clean them by soaking in degreaser and water for several weeks, changing the water every week. I like Simple Green ¼ to water.

Nice stone, good size for finishing, you will have to do many laps 100-200, but the edges are worth it.

The bevel will be a hazy kasumi bevel.

Oh, and Arks will eat DMT's

Try shaving off the edge, no leather.