Here is a link to an Article where a Lacquer Technician has been addressing the Coil dipping and coating methods with a Electrical Engineer.
It's a good insight if your dipping Coils as I am for preserving them.

https://www.ee.co.za/article/varnish-insulation-wire-enamels.html

The green Verdigris is a product of corrosion and likely moisture. It can also be caused by "foreign battery" causing a corrosive joint, exacerbated by the failure to coat a joint consisting of different material that will generate a potential difference.

I find that the bitumen or tar used on coils also increases the failure rate on coils & audio transformers, so it has sulphur, or some other contaminant in it. Some of the insulation lacquer of yesteryear was brittle, the modern stuff has a better temp resistance, & flexibility. So you design it to run hotter to melt the internal fuse.

Loose & rattly windings can chafe themselves to death. I always expect with early Kriesler sets to find two in eight with a speaker transformer that has not failed. & little Nippers with tarred OP transformers are not much better.

If a speaker goes open circuit (or not plugged in, with live transformer) , that can cause the transformer to "ring" and it will generate EHT: That will not help insulation with low dielectric strength.

Yes Marcc,
I agree . I have lots of Kriesler stuff, lots of parts and if you have a 60s Kriesler stereogram and both speaker Transformers are working you would be extremely lucky indeed! Whether there Rolas or Krieslers . This is why I'm looking into it and for fun.
But The Guy I spoke to who does the re winding on the vintage transformers down in Victoria said the same thing. That the old lacquers were not of good quality and they crack and corrosion starts and restistance gets high and higher as it gets worse.
So I have one more transformer to pull down just to have a look and see what I can see and learn by doing so.
The 2 good transformers I have ,,,I heated up in the oven for about one and half hours and I then dipped them twice in lacquer ,so Time will tell. But everything in this world seems to start with moisture, so being heated and sealed is going to help. If I can I would like to save and preserve this one original pair I have.
Pete

There is a lot in the US that sell bobbins for some of those Audio transformers: Not overly expensive if you buy more than one. Freight is normally the killer. Although some of that stuff imported by some purveyors of parts seem to be sold here at twice the inflated price.

There are some power transformers & I managed to fix one, where they have a metal strip embedded. in the wrap, that's wonderful until it comes loose & breaks the wire, or corrodes off.

There is a compromise between heat & how much Mar you can use. There is a batch of Philips transformers & my 123L has one where the windings come out the side. On the advice of a Motor re-winder I eased them back inserted motor insulation film between them & the metal, then in went the Araldite: So far so good & has had several hours of running at displays.

A guy in our radio club did rewind a transformer with some of the old stuff: Fail. All of it has been re-purposed by taking it to the re-cycler.

I will often coat antenna coils etc with circuit board lacquer, or Insulation Mar where there is an issue.

Follow up.
I have open up 5 Rola k9 speaker Transformers all out of different chassis and they all have one thing in common. Its the secondary that is going o/c not the primary, every one has corrosion in secondary windings with the primary staying in working condition.
I only have one Rola that is O/c in the primary out of a box full of Rola transformers!, All the windings in the secondary on every Transformer is rotten by some type of corrosion or some type of electrical corrosion, Its not green ,but the windings are all falling apart and I must say that the secondary is very well sealed off with wax paper and then plastic paper over that.
So I'm at a loss to why ? because I dont have the knowledge to know for sure ,but my feeling is its electrical issue doing this and not a corrosion due to them not being sealed because they are sealed well.

So I have 2 good Rola Transformers and I heated them up in the oven for about 2 hours and then I dipped them twice in a Tin of Lacquer to seal them right off .
Time will tell, but I dont have a lot of faith in 60s Rolas After going through this box of them ,but sealing them off is good idea just the same.
See pic of heated and dipped transformer

All the windings in the secondary on every Transformer is rotten by some type of corrosion or some type of electrical corrosion, Its not green ,but the windings are all falling apart and I must say that the secondary is very well sealed off with wax paper and then plastic paper over that.

Thanks for the follow-up. Very interesting.

In order for the copper to rot, the insulation must have been compromised by some means or another.

Here's an article from EA Feb 1991 in the Vintage Radio series by Rodney Champness that discusses historical reasons for failures in audio transformers in general:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&cad=rja&uact=8&ved=2ahUKEwihr_rQtbbgAhXGXCsKHQXtD-sQFjAFegQICBAC&url=http%3A%2F%2Fmessui.polygonal-moogle.com%2Fvalves%2FVR199102.pdf&usg=AOvVaw00nAZOB2r2M0abD_w8eFiz

Thanks G,
I will have a read and it solved my curiosity and I learnt a bit.
But at the end of the day its a Transformer and it either works or it doesn't in which case it goes in the bin.

The two articles are interesting. Neither mentions sulpher except perhaps indirectly in referring to the need to exclude acids and oils.

The critical point however is that Vintage Pete didn't observe green spotting, but dark stains on the tape. This could be copper sulphide which is dark, and is produced when intergranular corrosion occurs in copper when caused by sulpher compounds, in Pete's case in the tape.

The lacquer expert article mentions that one of the functions of encapsulants is to make the whole assembly a solid mass to prevent movement of the windings. Apart from causing mechanical damage to the lacquer insulation, applied static stress and cyclic stress can cause stress corrosion cracking (corrosion accelerated by the applied stress) and corrosion fatigue (corrosion accelerated by cyclic stress). Both of these mechanisms are intergranular. A lot of cycles build up at 50 cycles/second, ie 4.3 million per day.

I was once involved in an investigation into failure of tinplate drums containing printing ink during road transport - quite a mess. The investigation found intergranular cracking. We assumed that either or both of stress corrosion cracking and corrosion fatigue was involved. Stress from the weight of the ink on the thin tinplate, and cyclic stress from the bumpy ride, combined with corrosion caused by the ink. The possible solution was to lacquer the inside of the drums so the job was passed on to the lacquer chemists, as did the transformer wire makers.

Note, That as Lindsay mentioned in his post of his experience with coil issues while he was working at PMG. The tape was an issue of the break down of the coatings when they look into years ago.
The tape being the start of the corrosion would not suprize me at all! Because as many of you know I'm a cabinet finisher and if you put masking tape ,,,white or the blue on a cabinet that is finished in oil Varnish the tape residue eats and melts the varnish!!
Now oil varnish is the toughest of all the traditional lacquer/ varnish finishes. Its all about chemicals .What is safe to put on one finish will eat another type of finish depending on its base make up.
Eg, The Primary winding use cotton non adhesive tape and the secondary uses adhesive tape.

Nitrocellulose finish is an interesting one as it actually melts into the previous coat. The only issue with it is that, it starts decomposing from day one.

Secondary fail in audio transformers, I have not seen in several hundred radios? However there's not a lot of turns on them & they are on the outside.

Marcc,
I think its just these 1960s batch of Rolas that have the issues.
Because we have speaker Transformers much older than these that are still working well in many radios.
I guess thats why I was looking into it, but its beyond my knowledge . I wish I knew more about electronics, but I'm only a beginner, trying to learn as I go a long .I have spoken to a professional transformer rebuilder about the issues the Rola gets and he tells me its the coatings that fail and corrosion sets in

Varnish on copper wire was not the best in those days. Some of the manufacturing techniques led to the premature failure of thousands of all types of transformers though this was generally limited to certain models.

I recently watched a YouTube video of a robot in a Chinese motor factory winding some field coils into a stator. The robot's movements were quite rough by comparison with the same type of activity elsewhere. The varnish is flexible but very thin. It doesn't take much to scuff it.

The finer the gauge of wire, the more likely there'll be problems if it is roughly treated. If it is pulled too tight it'll 'draw' which will stretch the varnish and lead it to crystallise and flake off when heated. That's when issues come about.

Varnish on copper wire was not the best in those days

Yes, by comparison with today's insulation: "Modern magnet wire typically uses one to four layers (in the case of quad-film type wire) of polymer film insulation, often of two different compositions, to provide a tough, continuous insulating layer."

Tough is the word. It can be quite a chore to get it off for soldering purposes.

My wife is a electrical engineer, but she is not here at the moment, but will be back on the 6th so I'm sure she will point out where my thoughts are muddled and set my thinking straight as to where it has gone wrong and where I stuffed up!
Let you know what she says !