I've had the same thing happen with electrolytic etching: muddy appearance because it undercuts the masking. I thought it was a result of the thickness: since circuit copper is only a couple thou thick it doesn't need much etchant to be done, but trying to remove 12 thou (in the case of the pulsejet valves) or even more means there's plenty of metal on the sides of the cut for the etchant to attack, as well as just the stuff we want it to etch at the bottom of the cut, so it mushrooms. Which is a roundabout way of asking: do you think it would be a cleaner etch if you used ferric chloride on what appears to be roughly 0.025 thick material? At one of my previous jobs, my boss did his PhD on directional etching using laser-activated etchant, but that's a lot of research work.
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I thought it was a result of the thickness: since circuit copper is only a couple thou thick it doesn't need much etchant to be done, but trying to remove 12 thou (in the case of the pulsejet valves) or even more means there's plenty of metal on the sides of the cut for the etchant to attack, as well as just the stuff we want it to etch at the bottom of the cut, so it mushrooms. Which is a roundabout way of asking: do you think it would be a cleaner etch if you used ferric chloride on what appears to be roughly 0.025 thick material?
At one of my previous jobs, my boss did his PhD on directional etching using laser-activated etchant, but that's a lot of research work.