Question about etching tanks

I'm etching brass (0.005" thick) using a small tank I built at home. I heat the etchant (Ferric Chloride) to 82F using an aquarium heater and then
attach the brass with a "toner transfer" mask to a sheet of glass and put it in the tank.
The brass etches through in about 1 1/2 hours.
I have a problem with the etchant attacking the surface of the brass leaving it discoloured. I know that the toner is porous and lets the etchant through to the surface of the brass eventually, but since I'm using the other side of the brass as the finished piece this is not a problem.
However I can find nothing to use as a 100% resist on the other side. I have tried gluing the brass to a glass sheet but after about 20 minutes all the glues I have tried soften and start to let the etchant in.
Someone suggested using KK-2000 glue, but I find that this glue fails completely after about 10 minutes in the tank, and releases the brass.
Enamel, polyurathane, and paint all fail and let the etchant in eventually (especially where the etchant "drills" through from the other side).
Does anyone have any idea how to protect the brass completely?
Thanks, Steve
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