im proposing to use copper sulphate to check taper-fit, lobing and such.
instead of messing with blue, spreading it with various methods trying to achieve an acceptable distribution, you have a jar with a copper sulphate solution that can be used forever. you dunk the taper in there for a few seconds (depending on temp.) and get an extremely uniform coating of copper. sub micron thickness, if you want. i managed to chip off a piece that i consider very thick and it was 3µ. the copper layer is soft and rubs off easily. what stays behind is a faint glaze, like after using a bronze brush on steel.
instead of messing with blue, spreading it with various methods trying to achieve an acceptable distribution, you have a jar with a copper sulphate solution that can be used forever. you dunk the taper in there for a few seconds (depending on temp.) and get an extremely uniform coating of copper. sub micron thickness, if you want. i managed to chip off a piece that i consider very thick and it was 3µ. the copper layer is soft and rubs off easily. what stays behind is a faint glaze, like after using a bronze brush on steel.