I also chalk my files before use, then use a file brush and 90 percent of the white metal comes right off. I learned this trick working on the lathe, but it works for the miniatures also. I have files that are 20+ years old and still going strong.
When I used to use the files on lead figures a soaking in mercery would take the lead right out, but my supply of mercury is about gone, and it is really not a very safe thing to play with, and the mercury becomes contaminated. I have not tried it with the lead free metal the figures are made of today.
Dale