I cut a lot of firewood, and when my chains get dull, as evidenced by smaller and smaller chips being thrown, I sharpen in the field. Get the correct round file and a file guide, a pair of heavy leather gloves, and go at it. Should take about 5 minutes. Lots of on-line assistance. The trick is the right angle for the file, which is where the file guide can be a big help.
Phil Millam
Winthrop WA