Here is a follow up to etching with ferric nitrate. Sandra Noble Goss uses 4oo ml distilled water to 300g ferric nitrate. She finds it etches silver in 2 to 7 hours depending on age of the solution. She also finds the PNP BLue resist holds up to this. Agitation can help too.
She has had better luck than I 've had, or perhaps I am just not patient enough. In my experiments I was etching a very thin layer 2 hours hadn't begun to do it with a new solution and the resist was not holding up. The crystals absorb moisture as soon as they are exposed to air and rapidly become ineffective. I truly wish this friendly substance would work consistently, but it isn't the nature of the beast. I have resorted to a dilution of nitric acid for silver, sold by Rio Grande. I have access to a fume hood with excellent ventilation at the art center were I teach. Using this in one's own studio, or home is a matter of personal choice. Everyone should do their homework about the hazards and clean up protocol, before making the decision. I have decided against it.