Good news; I was able to reduce contact angle to about 30...50 degrees throughout the whole sheet and many times in a row. Water behaved completely differently now on the surface, forming large (~1 cm) puddles instead of small drops. This contact angle should be really enough for coating, but after so many failures I'm not sure yet . I made a test coating again and will report soon.

The key was a new anode electrode made of metal mesh to increase its surface area. And, the surprising part, which I accidentally found, was that no matter what, the wrong side of the sheet seems to get the treatment. So, the side facing the anode (mesh) does not work, but the side facing the dielectric-coated plate (cathode) shows a great hydrophilicity. Changing the polarity didn't change this; the side facing the plate was treated. Oh well, this doesn't matter really, if it just works, but I was surprised. The voltage has still to be very high, 18 kVrms, despite the patents speaking about much lower voltages.