When I was an tool and die apprentice one of the things that was taught to the machine repair trade was a thing called scraping.
It was the most accurate method of achieving flatness, parallalism, and squareness and on a a machine bed.
No mechanical pressure or heat from a grinding wheel, just a dial indicator and hand work to get the accuracy you need.
You may actually see scraping marks on a lot of mill and lathe beds, and not realize they were all done by hand.
Scraping involves a scraper,a granite table and a dial indicator and a lot of time to check your work.
There is a video on youtube called "What is metal scraping?" by NYC CNC. it will explain scraping and why it is used.
I believe there are other videos showing the technique of scraping on youtube as well.
-George