Another newsprint stropper here! I can often refine a flagging edge with 100 strokes on colored newsprint. Going on the theory that maybe the colored inks are slightly abrasive, I usually use Sunday inserts, comics, etc., and try to use mostly greens-maybe there's some chromium oxide in the ink? Could be just voodoo, but it seems to work for me.

For my next experiment, I am going to compare edges between conservative vs. liberal-leaning newspapers.