As mrgraham never came back to this I thought I would let everyone know HCL does work.
I have this problem and google finally brought me to this thread. I just happened to have some Concentrated Hydrochloric Acid from about 30 years ago so gave it a try and it has remove virtually all of it, just a light ghost is left.
Method used
NOTE: You need protection - e.g. gloves, mask (there will be fumes), old clothing
I used an old cloth to apply the acid.
Apply a dab of acid to the cloth.
Coat the glass squares with the acid (best to avoid the leads as it will clean them and probably make the issue re-occur)
Leave on for a few seconds
Rub square with dabbed acid cloth.
Clean off acid with plenty of water.
Dry with a clean cloth.
