Crosshatching is a way to, not only flatten a part, but to see where the problem is. I just finished a knife whose guard is shaped such that I had to work part of it flat by crosshatching.
In a dry fit, I could tell there was a problem, but could not tell exactly where to remove material. So I got the scratches all running one way, then took a wide backing stick and changed the grit on my sand paper so I could see the difference better and sanded perpendicular to the main scratches. The low and the high showed up easily then.
It was then a matter of concentrating on the high areas till everything was even and flat, by alternating between the two directions.
Here I'm pointing at the low area that had the larger initial scratches.
And here I am pointing at one of the high areas that I had to take down a little. My backing stick spanned the whole area to bring it down all together. Then I also had to blend the curves into the flat area.
I thought this might interest some of you who make knives.