I'm a long-time lurker, first - time poster on this forum. Im seriously considering a set on Jimmi's R1 armor.
I worked with needle files and a Foredom (which is basically a much more reliable version of a Dremel tool) every day for almost a decade - in the custom-manufacturing industry (jewellery). An area of 1mm x 1mm was actually quite noticeable in these cases.
One thing to remember is... to USE A VERY LIGHT TOUCH with your file, and let the "sharpness" of the tool (i.e. crustiness of the diamond bits) do the work for you. Avoid the temptation to use the force of your muscles to get the job done. (You know how you need to use a lot of force to use a dull knife to cut a tomato, or a loaf of bread - and still the knife might just crush the food? Well, then you need a sharper knife.)
Using more force puts pressure on the material in ways that can torque it and give you undesirable results - like breakage ...whereas practice at using light pressure (yet varying between tool surfaces that cut very deeply versus tools that cut very shallow) will yield MUCH more predictable results.
In regards to the picture that 11b30b4 posted above, I would actually recommend diagonal strokes - from either corner - to ensure your vertical lines are kept straight, from corner to corner.
Additionally, once you get comfortable with the tool (the file), always work from the most difficult (or hardest-to-reach) areas first, then to the easiest areas.
1. For example, it appears like the top corners of the vent slits don't line up. I would ensure those are perfect before finishing off the long strokes in between the top & bottom corners.
2. Furthermore, if you notice that a tool slip will cause a blemish in area you have already completed, that's a problem & you've just made more clean up work for yourself. So try to figure out ehoch surfaces will likely receive some tool-slip marks & finish those up LAST.
