I want to replate the faces of several antique mortise latches. Like the knobs and plates my old house has the usual mixture of downstairs latches faced with brass and upstairs latches which are faced with plated steel, and the plating on the steel is deteriorated or missing to various degrees.

Each face is swaged onto a cast iron case half. I am afraid to try to separate the faces from the case halves.

I can easily remove the layers of paint from the face itself ( a previous owner painted anything that didn't move). My problem is the cast iron case half. It is covered with a thick black paint, japanning I assume. I can remove most of this coating, but I think it is nearly impossible to get it all out of the cracks where the two pieces join. I am afraid of contaminating my brass plating solution if I immerse the part with this remaining in the cracks.

Any suggestions on how to clean this out? Soak in lye?

Or perhaps some kind of resist to seal it during plating which is easily removed afterward?