What material is used for masking out areas that are not required or not wanted to be rusted when going through the rust blue process.
Looking at my 1916 DWM it is clear that the internal areas of the frame etc are still in the white, whereas the outside surfaces are (obviously) blued.
Some kind of material must have been used to prevent rusting of these internal areas. Some kind of laquer maybe?
I would be interested to find out from some of the rust bluing gurus out there.
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