• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Alkyd varnishes for transformers: specialized vs regular boat varnish

Now I'm wondering, due to air dry alkyd varnishes like Elantas Elmotherm 009-0008 getting more and more expensive, are they somehow superior to regular alkyd boat varnishes you'd find at regular paint shops? And to what extent? Drying time? Anti-corona additives? Could regular alkyd varnishes contain offensive to the transformer chemicals?
Don't know about that but any varnish as long as it's not corrosive is better than nothing. The stuff I've used is Ultimeg 2000/372 but this is Xylene based which plays havoc with the adhesive on tape turning it into slippery mush. I'd have thought any decent water based varnish will surfice to secure windings, laminations etc on a "normal" tfmr, say with voltages present of around 500v.

Once you start hitting 1kv though a varnish purposely designed for electrical applications would be needed. I've tested my hand wound toroids with no varnish used up to 10kv with very good insulation results. I've used the same test on vintage varnish impregnated tfmr's and they haven't tested as well.

Lastly I've recently took apart a Tektronix 545B mains tfmr that used no bobbin and varnish impregnation. They'd also used what looked like car body filler between the turns, this may be an alternative to or an additional method of insulation.

I'm aiming for projects bellow well 1kV and they're already having a pompous amount of mylar/paper insulation between high voltage potential layers, so in this case I'm not requiring super insulating capabilities from the lacquer. The main purpose is to seal the insides from moisture and securing the windings. My main fear was some sort of chemicals inside classic varnish that could be corrosive. Thank you for the thorough reply!
That makes your job easier in that case as your aiming for fixing everything in place rather than insulation. Perhaps you could make a few trial samples, say use some offcuts of wire wound on a simple former & coated in the intended varnish, then tested or examined visually. You could do simple resistance tests on samples after being dunked in water, a Megger or similar would do the job. I use an old Airmec Type 251 Ionisation Tester, you can listen to any dielectric breakdown, here's a video of me using it -

Are you vacuum impregnating or coating as you go? It might be an idea too to reach out to the makers of the varnish and see what technical data they have. Lastly what do you use for insulation? You mentioned mylar, have you used Nomex & onion paper or similar?

Amazing machine you got there! Thanks for sharing the video!
Yes, I'll be definitely testing the lacquer with a few wire scraps first. Not sure how long is enough, I usually leave the samples soaking for 48 hours. So far, I haven't had problems from usual alkyd solvents, such as turpentine, xylene or toluene, including class 1 and class 2 enameled wire.

The transformers will be vacuum impregnated. I could just pot them, but there is paper insulation inside as well and I prefer filling its fibers with something to kill the hygroscopicity of the paper. I'm using paper only insulation between low voltages, sub 100V and paper/mylar dielectric between 450V layers. Note that the wire class is 2, polyimide-imide with a rating above 1kV

There won't be enough time for the potting compound to migrate well within the fibers before setting, so I prefer a long soaking vacuum/pressure impregnation prior to the potting. It doesn't have to be necessarily alkyd varnish, it could be shellac or something else, but for this project it has to be within budget and preferably fastest to cure. I'm tempted to try shellac, because of the lower level of... stink.
Alkyds are oil-based and mostly impervious to humidity and water-based contaminants.
Shellac is soluble in polar solvents and can absorb humidity. In addition, it is a natural product and has some variability. For furniture, it can be seen as advantage, since it gives an artisanal aspect to the object, but for technical applications you want as much consistency as possible
You can try 2-component low viscosity lamination epoxy (with long curing time and good wetting properties) used for sheet composite manufacturing (drones, etc.). There are tons of manufacturers and products to choose from. I made sheets with outstanding mechanical properties just with 2mm presspan cardboard and 2-component lamination epoxy.
BTW, they cure on their own and don't need open air to harden.
  • Like
Reactions: 1 user
Ask at a motor rewinding shop, they will tell you what is most suitable and available in your area.
They might even sell you some.

I paid 50 cents last year for a 200 ml. bottle of Class F (135 C) insulating varnish. Schenectady Beck was the maker, considered good quality here.
Ask at a motor rewinding shop, they will tell you what is most suitable and available in your area.
They might even sell you some.

I paid 50 cents last year for a 200 ml. bottle of Class F (135 C) insulating varnish. Schenectady Beck was the maker, considered good quality here.

We have a seller for such specialized lacquers, however the prices increased quite during the pandemic. But I'll consider looking for import of worldwide manufactureres and comparing brands.
@NareshBrd, I already have potting compounds in stock. But the idea is to get the transformer soaking time to have very well impregnated windings, with air and moisture removed. Potting compounds usually begin to gel within 30 minutes and there's not enough time for it to seak well within a deep and tightly wound coil.
@50AE, do you varnish all transformers or only those which parasitic capacitance is not a key parameter?
BTW, one of the engineers I know uses quick dry BF-2/BF-4 soviet era glue to varnish and fix winding on layer to layer basis. Unfortunately due to inclusion of one of its componenets called Dibutyl phthalate which considered toxic it is not sold in EU anymore.

Any business motor and/or transformer winding house should be registered/approved for conformal coating under IEC NEMA specs for any windings i.e lacquers now be Urethane based varnishes. Ask one nicely and provide a jam-jar.
On my back benches doing winding work as I often do, some decades ago for quick work I used Yacht varnish polyurethane based was an alternative and I used it for general applications, but may contain anti fungal and other agents agents, thus may not be effective dielectrically in the longterm. The last one wants is a lacquer that ends up peeling off over time.
For those in Europe an example is https://www.itwcp.de/product-1211411-en.html
I have impregnated many windings with this. But beware doing valve output transformers. An overnight soak will certainly ruin and increase the interwinding capacitance, so the adopted practise is after the final laminations installed, a quick dip just to seal the winding ends.
You need how many kilos?
The stuff is about $10 per kilo, and shipping by air is not allowed for liquids.
I think you will be happy with 2 kilos.
There exist slow set potting compounds, I dunk them in varnish, on load, works as a viratory feeder.
Then drip dry excess, and use light load to heat set the varnish.
Some winders use 40W filament bulbs for that.
I also considered taking a look at single-component poxy or polyester varnishes. They're wet until cured at specific baking temperatures. Some work at 130, others begin at 150C.
Yes, of course, impregnating will increase interwinding capacitance, especially for low-density high-air content dielectrics such as some types of paper. That's the idea - taking the air out, replacing it with higher epsilon dielectric.
Yes, I've impregnated some output transformers with the capacitance increase prediction. If a specific paper haд an epsilon of 2.5, it has jumped to 3.5 for me.
However, securing windings well matters the most in power transformers, especially where a high-leakage relative coil together with a high capacitor value input filter does make the coil rattle. Securing the windings as monolithically as possible, and sturdy mechanical coupling of the whole coil to the core is mandatory for noise-free operation, at least for my kitchen.