This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The best plan to follow is probably this - First brush the casting thoroughly - scrub it - with a hard brush; this will rub off the loose sand; then take an old file, and file away steadily at the skin till you come to a surface of pure metal. Having by this time removed those parts which spoil files, the "old file," with which but slow progress is made, can be changed for a better one, and the best, as well as the most economical, will be one which has been used for filing brass till it has become too much worn for that material; such a file is in first-class condition for working on cast iron (when cleaned of its sandy skin), and when worn out on that it will serve admirably for steel.
When it is necessary to file up a small surface - say 2 in. or 3 in. square - the file must be applied in continually changing directions, not always at right angles to the chops of the vice, as, though the work might be made perfectly straight in that direction, yet there would not be any means of assuring a like result on that part lying parallel to the jaws. When the surface is fairly flat, the file should be applied diagonally both ways; thus any hollow or high places otherwise unobservable will be at once seen, without the aid of straight-edges, etc. This method of crossing tin-file cuts from corner to corner is recommended in all cases, and the file should invariably travel right across the work, using the whole length of the file, not just an inch or so at some particular part, as is too often the case. When in use, the file must be held quite firmly, yet not so rigid that the operator cannot feel the work as it progresses; the sense of touch is brought into use to a far greater extent than would bo imagined by the inexperienced, and a firm grasp of the tool, at the same time preserving a light touch to feel the work, is an essential attribute of a good filer.
In filing out mouldings and grooves which have sections resembling, more or less, parts of a circle, a special mode of handling the file becomes requisite. The files used are generally rats'-tails or half-rounds, and these are not used with the straightforward stroke so necessary in wielding the ordinary hand-files, but a partial rotary motion - a sort of twist axially - is given to the file at each stroke, and this screw-like tendency, given alternately from right to left, and vice versu, serves to cross the file cuts and regulate the truth of the hollow.
With regard to cleaning tools which have become clogged up with minute particles of metal, dirt, and grease, files which are in that state are not fit to use, and the following directions will enable any one to keep them in proper order. The most generally used tool for cleaning files is the scratch brush; but this is not very efficient in removing those little pieces which get firmly embedded and play havoc with the work. File cards are also used; they are made by fixing a quantity of cards - such as a pack of playing cards - together by riveting, or screwing to a piece of wood. These file cards are used in the same way as the scratch brushes, i.e. transversely across the file in the direction of its " cuts," and though neither tool produces much effect yet they are both often used. When files have become clogged up with oil and grease, the best plan is to boil them for a few minutes in some strong soda water; this will dissolve the grease and, as a rule, set most of the dirt and filings free; a little scrubbing with an old tooth brush will be beneficial before rinsing the files in boiling water and drying them before the fire.
These methods will prove effective in removing the ordinary accumulation of dirt, etc, in files, but those " pins " which are so much to be dreaded when finishing work can only be removed by being picked out with a scriber point, or, what is better, a piece of thin, very hard, sheet brass, by means of which they can be pushed out very ' easily. These "pins" may be to a certain extent avoided by using chalk on the file, if it is used dry, or a drop or two of oil will sometimes help matters.
With regard to finishing filed work, such as has to be made particularly presentable to the eye, there are many ways of polishing, burnishing, etc, but, properly speaking, that is not filing. There is much beauty in well-finished work, perfectly square and smooth, as left by the file, untouched by any polishing materials; in such work the filing must be got gradually smoother by using progressively files of finer cut, and, when the work is deemed sufficiently finely finished for the purpose, the lines should be carefully equalized by " draw-filing," that is, the file is held in both hands, in a manner similar to a spoke-shave, and drawn over the work in the same way, producing a series of fine parallel lines.