I engraved rhodium plated Brass using a dry moly.
Rhodium is as hard as anything. Tried it with Cermark, and was incredibly dissapointed.
But had great success with dry moly. Researched both rhodium and Brass heavily, all responses were pretty conclusive. Cermark will not work with either of these metals.
I actually won this job over two local competitors, both of which did a sample using a rotary and diamond drag engraver.
I managed to achieve some depth surprisingly, can feel it when you run your fingers over it, not to mention great contrast. Leaves like a matte black finish over an incredibly glossy metal. Couldn't of been happier. Only challenge I had was removing the excess dry moly from the award, had to use methyl ethyl ketone or MEK as a DNA or hot water wouldn't work. Even then it was tough going, and not scratching the rhodium was definitely a challenge.
But well worth it for the $$$$ on this job.
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Epilog Fusion 40 (60w)
CAMaster CNC Router.
Flame Polisher
Range of sanders
Drill Press
Table Saw