I'm getting ready to start the repair of an old neon artwork that is assembled as a skeleton sign, and I noticed some of the tie wire has a white residue at a few of the contact points where it wraps around the neon tubes. Does this mean the metal of the tie-wire is breaking down? The sign also buzzes loudly from around these areas and gives off an ozone smell. Just curious what is causing this and why it is only happening at some of the tie-wire points and not others. Could the tie-wire be causing the buzzing? Should I switch it out for fresh wire?

Yes, it is building up a bit of a capacitive coupling (the ionized gas in the tube effectively being a plate and the wire effectively being the other plate, with the glass and blockout being a dielectric between them) and the wire or the blockout paint is being oxidized by the acids created when ozone combines with the moisture and nitrogen in the air, forming nitric acid.  

This normally does not happen much, but since you mentioned the buzzing, it is likely that these tubes are very old and have a lower effective fill pressure due to the gas being cleaned up over many decades and this will cause the transformer to drive it at a higher voltage which will exacerbate those sorts of things.  If you wanted to try an quantify the situation, you could consult a footage chart to see what transformer should run it, then use a magnetic transformer and a milliamp meter to see where it actually falls in terms of being under or over loaded and that would give a clue about the health of the tubing in terms of gas cleanup......but since you are getting that extra buzziness, it is a clue to that being an issue.  Another thing that will really aggravate this, even in a new tube, is using metallic tie wire to join sections of tubing together that have a high potential difference between them--such as the doublebacks at the ends of a unit that happen to fall close to one another as in a circle.  You may want to switch to silicone and/or a bit of cork or rubber in the gaps or a nonconductive tie.

Thank you for the reply Robert! Yes, the tubes are old, I think from the 1980's. And I do think the footage of the entire piece is on the high side, and plan to clock out the actual footage when I trace out a pattern of it. It is a very large piece, about 4' x 4', consisting of 5 total units: 2 green, 3 neon orange, all 15mm. One of the green tubes is broken, so I'm remaking that.  I do plan to replace the corroding wire in those spots, but am also thinking it might be best to split off the wiring so that it can be hooked up to run with two transformers, instead one, hoping that corrects the buzzing. I really don't want to reprocess tubes that are on their way to going bad, so I'm hoping that's not the issue. We'll see!  I'll come back and post an update after I troubleshoot and report my findings. :)

Just came back to edit this message and add a link to a video I took of the buzzing.

https://drive.google.com/file/d/1TeOiRHH7kYYU30Mjw6uCxczoe3Jitsmw/view?usp=sharing