I fixed an air compressor that blew gaskets by filing the warped block flat after pouring wax onto the pistons to catch the debris. It was too tall for my surface grinder. I ground the head flat and used it with bluing to see where to file the block. That problem is solved but it still has others...
I have a new coarse/fine stone that's only used to take the burrs off machine tool tables. The one I sharpen knives and plane blades on isn't flat enough any more. SiC paper on a flat block works quite well if you can find the flat block. I've used 1-2-3 blocks and a ground angle plate salvaged from an old rusty fixture. The area between the rust pits is still very flat.
In general you can work to whatever accuracy or flatness you can measure, because after measuring you know where to correct. I fitted loose scope bases to another shooter's rifle at the range by smoking the bases with a lighter and scraping the contact marks down with pieces of broken glass. He went from off the target frame to about a
4" group.Jim Wilkins