During my apprenticeship, at one stage I was making loft plates, which involved cutting and filing a profile to (ideally) half way through an 0.008" thick line, the material was generally 1/8" Al Alloy although some were 1/8" or 1/4" gauge plate. The inspector wanted to see a draw filed profile as they generated less drag to the router, some of the profiles were 8' (that's 96 inches) long and gently curving. This enabled the development of draw filing skills to quite a high level!!
Even now, over 40 years later I can still file to a line and (split a thickish one!) nice and square etc etc, I just don't do it very much, because I can.........., machining is less effort, just twiddling a handle!!
cheers
Bill Pudney
Adelaide, Australia