Jim Matthews
Yours looks pretty clean, already.

Test for flatness with a ruler longer than the stone and a flashlight in a darkened room.

I would not subject this lovely example to extremes in temperature, when mechanical "scrubbing" should suffice. If you don't have a plate for use with lapping compound, float glass is good or an I used cast iron plane body.

Once you save up some funds, the Trend diamond plate (300/1000) is well received.
That's a good way to keep Arkies flat.