Sean, looks like you really have done a lot of research and testing.
Wish I had the zinc kit so I might be able to assist you.
Concerning the current: I realize that one should adhere to the recommended current settings per square inch and for the most part when I just set the voltage to the specified amount the current draw on the parts is just about where it should be.
Maybe I have just been real lucky.
Like I said the only problem I ever had was with the acid copper.
Every once in a while I might get a little burning on a part edge, however, I have learned by experience how to hang the part, anode placement or using cheater wires to prevent it.
I just don't have a clue what is going on with your zinc, since you appear to be doing everything correctly.
I understand what you are saying about the conductivity of the solutions and only being able to control that by chemical make up or alteration.
My CopyChrome solution is not very "conductive" so that is why I have to run it at 4.5 to 5.0 volts.
Like you said, the zinc solution may very well be very conductive and perhaps placing the part as far away from the anode may (at this point) be the only viable alternative until you do some more testing.
Try Caswell's PRS site again, maybe they can help.
On a side note, I always run my solutions at room temp (in FL that is about 95 degrees in the garage during our (
month summer) and in the winter temps (55 degrees or so) the only difference I see is a "slower to plate" issue, so sometimes I put the heater in to raise the temp to about 80 degrees.
Good luck, and keep trying!
George W.
Orlando, FL