Incidentally, you are trying to file out the crack so that not only is the crack completely removed, but ALL defects in the file-out area are completely removed !! Don't just stop when the crack is gone, because the file will leave scratches which are stress risers!
Buff the area with 1000, 2000-grit sandpaper, and use nevr-dull or simichrome or a flap-wheel/dremel to finish the job beautifully. After that, I'd get a red magic marker and mark & wipe the area a few times as a poor-man's dye penetrant test.
Precision polishing will raise the fatigue limit of the piece. In the M.S. thesis below, polishing raised the fatigue limit of some aluminum samples by about 50% (figure 18):
http://fcp.mechse.uiuc.edu/
When you can no longer see any crack and when the area shines like a mirror, you are done!
- Don Gillies
San Diego, CA, USA