I'll add to Robert58's post, clean the file often with a file card and refill with chalk or soap stone. I load the file with soap stone, both sides top to bottom. Start the draw filing process near my dominate hand, make a few cuts, flip he file make a few more, then move the file to expose a section of the unused loaded file. Continue flipping and moving until the entire file has been used, then clean and reload with filler.Be careful not to tip the file or apply greater pressure on one end or the other, as this will round the edges.Wipe the barrel with a clean cloth often to remove metal filings. When your finished drawfiling, degrease the barrel and don't handle it again with bare hands, wear clean cotton gloves or latex/rubber gloves.
For best results use a new file or at least one that doesn't have any rust or metal embedded in the cutting surfaces.