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Sorry! I forgot to state that the granite is the base that you put the sandpaper on so the paper lays absolutely flat.
Wet the granite so that suface tension of the water will hold the paper in...

I have a relatively new Corian counter top which is around 3/8" thick. It seems stable enough but when when applying much pressure on it I can feel it flexing. I used that surface to lap on for...

You do really need to lap that Norton. They are really rough surfaces when new.
I found that I had to lap more than just what was needed to get it flat.
Once I got mine flat which I proved by...