Bookmark the PDF link, share this article with your machining club, and consider digitizing your own old shop manuals. The art of hand scraping survives only if we practice and teach it.

Hand scraping is the process of removing minute amounts of metal (often 0.005mm to 0.07mm per pass) to achieve high-precision flatness and alignment. Technology of machine tool maintenance - Pure

by Edward F. Connelly (often considered the "Bible" of the industry).

Using "Prussian Blue" or other spotting compounds on a master reference surface (like a surface plate) to highlight high spots on the workpiece.