I think this is going to be a disaster. I have experience with Ferric Chloride, at least, as a copper etchant. Your problem is that you may initially get just what you are looking for, and then, 2 weeks later, you find the manifold in pieces, with big hunks missing. The trouble is getting the etchant (whatever you use) to STOP when it is done. Cast iron is notoriously porous, and the etchant will hide in the pores and do mischief over time. If you do try Ferric Chloride, you will need a very aggressive neutralizer to fully inert any remaining etchant hidden in pores of the metal. Alkalis are probably going to be good for that.
My other reason to suspect problems is the grain structure of cast Iron.
It has a very
loose grain structure, with lost of inclusions. Most etchants will
follow grain boundaries,
preferentially working BETWEEN the grains. This leads to faster
etching, but very
UNEVEN etching! In other words, the surfaces start off somewhat smooth,
very jagged as the etching progresses. Instead of moving a wall back
the general contour, it will turn the wall into swiss cheese, or moon
something like that. Maybe this will still get you what you want.
18 years ago