That does sound disturbing. Perhaps , like my SCT, a few scratches and defects in the finish are nothing one can see . The Mel Bartels site suggests that you can live with final scratches.
There is an Indian scientist and mirror maker who suggests that every grade of grinding material he gets requires testing with a method he describes.
Maybe I should try that if there is a risk of problems.
https://sites.google.com/site/diy100atm/getting-started