Hello Marc, File etching goes better in this acid solution It etches
more to the deep. than pure Nitric acid. Works also well on burrs.
Clean the file in a hot washing soda bath with a brush, or with an
other way of decreasing.
Than put them in the following mixture: 7 parts water 3 parts sulphuric acid 1 part Nitric acid.
In a few minutes ready. (remember first water that acid) Neutralise
in washing soda again.