On another tool forum, dealing in woodworking tools this seems to be one of the preferred methods. Lay the say on a flat surface, well supported. Take a new single edge razor blade, scrape as much of the heavy rust off as possible. Then lightly oil the surface, light oil, even atf. 10 wt. is good too. If it is real rusty I will start with a 320grit piece of paper, on a backing board. Finger tips here with push down un evenly and destroy the etch. When something comes into view. Switch to 400 grit, the oil quickly turns to red rust, stop and wipe it dry, reoil, keep lightly sanding, whatever etch is on the blade will come up. Some guys use different acids, some use evaporust, which I hear works great. I have access to lightly used sand paper so use that. Moving the saw blade around letting the light hit it at different angles usually reveals anything thats left.
