Old patents I read almost a year ago were using, for example, seven electrodes in a circle, the web passed them all. This of course allows higher speed for web than one electrode, and increases evenness. I can well imagine how big problem ozone is on bigger units, as it smells quite strongly even when treating just one 21x30 cm sheet so that it is completely treated.
Some patents talk of using a metal net instead of single wire, for larger surface area and many points for discharge.
Luckily, I find currently no need to make any big unit that would use rollers to move a long web, or do a quick job with single pass. This sheet version with manual electrode is just fine to treat, say, up to 1 meter long sheets.
I think that, from the possible pitfalls mentioned, the metal from electrode and possible fogging in case of overtreatment may be the ones I should be aware of.
I thank you PE for pointing out any possible problems; by no way I take that as any kind of discouragement! I'm very happy about the positive response I get from all of you, and all the ideas I get.
Actually, I'm currently winding a new transformer; I got the previous one to arc out, but that was all due to my very clumsy handling that caused some of the insulation came off very badly. Winding a decent HV transformer is possible, but quite difficult and tedious, especially if you want it to be completely corona-free to last for years. Mine is not, and corona discharges happening inside can shorten the life, but in my very intermittent usage this is not a problem.