My idea takes the warp out of thin plates for grinding them flat. When the warp is more than 0.001 in., I place shims under the plate. But when it's less, I use a paint marker.

Paint from the marker fills-in the warped areas, acting as a super-slim shim. But, a word of advice is to always let the marker paint dry before mounting the workpiece on a grinding surface plate.

Al Chase
Rochester