If the rasps/files are not too badly worn, they can be sharpened by soaking in hydrochloric (muriatic ) acid. The acid eats the metal fairly evenly resulting in new sharpened edges on old blunted teeth. Some PVC drain/waste/vent pipe with an end cap welded on makes a convenient tank for the process. It should be carried on outside or in well ventilated areas as hydrogen gas and acid fumes are given off in the process. Also, the acid has a taste for human flesh so wear protective clothing, gloves, and eye wear.