I haven't tried the stronger acids. The vinegar works and is very cheap. I leave mine in for a week or two. So longer than what guys are getting with stronger acids.
Whether its a flat tub or a standing tubular container, use a container just large enough for your file or files, as this reduces the amount of acidic solution needed to fully submerge the files.
Keep a wirebrush on hand if you are restoring old rusty rasps or files. This helps remove the surface rust after its been acid bathed for awhile. it is also a good way to apply a coating of oil that is not excessive.