I used very flat diamond steel plates. Flatness of the stone is not the problem. Unlike water stones that build up swarf that can conform to slight hollow and polish into it, the diamond plate shows every imperfection. It will just take some more work.
The spots are actually a little surface rust. I must have been a bit frustrated and did not dry and oil the chisel when I quit flattening the back. I used some steel wool to remove it. It oiled now