Removing transformer varnish

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Does anyone know of a solvent that can be used to remove/dissolve transformer varnish? I currently have a whole pile of salvaged EI laminations but I haven't found a way to clean them up. Some people recommend burning or baking the varnish off but I don't want to alter the metallic properties in any way.

Also, are these solvents safe for use on nylon bobbins and enamel wire?

Any advice would be appreciated!
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I've always had a problem with getting varnish off of wires and whatever too. Usually would up with a wire brush or chipping/scraping with a sharp edge - sandpaper - files etc. At work we used chemicals - but they were strong smelling and not environmentally friendly at all and consideration about getting rid of the used product made it's use a show stopper.

I decided to revisit the issue and came across this stuff - looks interesting and might be worth checking out. :scratch2: Safest Stripper Paint Varnish Remover Qt Paint & Varnish Remover: Sports & Outdoors

I have not used this stripper and thus cannot vouch for it. If you wind up giving it a try I would love to know if you like it or not.
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Dichloromethane (DCM) is extremely efficient at the job, it will take tens of minutes or at the most hours instead of days to work, but it will remove the enamel of the wire, and damage most plastics.

It is highly volatile too, and you need to enclose the solvent and the "target" in an airtight container, otherwise it will evaporate before the job is complete.

As all chlorinated solvents, it is toxic, harmful, etc, etc.
I'd try whatever solvent you intend to use on a scrap piece of enameled wire before you start. I don't know what sort of enamel is used on wire, but lacquer thinner is very active and softens many plastics. Acrylic lacquer thinner is even more active than the "straight " variety that can be purchased at many hardware stores. If you intend to soak the transformer, I would start out with something relatively benign such as mineral spirits, often labeled as "paint thinner" before trying something more potent.
I was successful in the 80s at completely removing the varnish from a bunch of transformers by dunking them in a coffee can full of paint stripper for ~24 hours. The varnish bubbled up and rinsed right off. The coating on the magnet wire was not affected, but I didn't reuse it anyway.

These days it seems like they've removed a lot of the methylene chloride from paint stripper, as it doesn't work anywhere near as well as it did back then ( nasty stuff, I can see why they'd want to remove it). Xylene is one of the components of your standard transformer varnish, but that's not enough to get the job done, either, possibly due to the chemical changes that happen in the varnish when it's baked. Straight methylene chloride might be your best bet, though I don't know where you'd get it, and you'd want to strictly keep it outside of the house if you managed to get some. Don't get any on you - it burns like the hinges of hell, and it decomposes inside your body into nasty side products including carbon monoxide.
Removing transformer varnish-nasty chemicals

Strong, by now illegal chemicals work somewhat, but not to fully recover the laminations. I've only done this a few times to reproduce a fried transformers turn ratio and layering construction. Near impossible to get them clean enough for reuse. The magnetic path will never match the design using new stock. Residual gunk will not allow the lams tight coupling. Best bet is to try finding new stock.
I recently took afairly big, old transformer apart by boiling it (on a low simmer) in ordinary drain cleaner (sodium hydroxide). I cooked it for several hours, turned the heat off and left it on the stove until the next day. The liquid solution we buy in Denmark holds 40 % weight/volume and that should probably be diluted to about a quater to half that strength. I used the solid stuff and added that to some water in the pot.

WARNING: People who are chemists (I am) can skip this paragraph. Others should know a few things about this chemical before working with it: 1) Always wear safety goggles! (This stuff is nastier than battery acid in terms of burns. It is also much harder to rinse off as it turns the fat in your skin into soap and it penetrates much deeper). Rubber gloves and a plastic apron are prudent measures, too. Your prime concern is not to get the stuff in your eyes or face. 2) It evolves much heat when poured into water. Don't add too much too fast or it may boil violently and spray drops on you. Add the concentrated drain cleaner into the water. Never do the opposite - the heat can get out of hand. Your prime concern is not to get it in your eyes or face. 3) Do NOT use an aluminum pot. Drain cleaner, especially when hot, attacks aluminum ferociously. The pot will dissolve and the reaktion evolves hydrogen (explosive gas). You can safely use most other metals. And glass lids are also safe to use. Steel or stainless steel and glass is left unharmed, though very clean. 4) Sodium hydroxide is not poisonous. It burns, so don't drink it, of course. But, once the pot is rinsed it will be perfectly safe to use for food afterwards. Small dilute traces of it will not harm you in any way. 5) It does not emit any harmful vapours or gasses (if you keep it away from aluminum). If you smell anything, it is just the laquer getting broken down.

(Back to the subject): The laquer dissolved completely on the outsides of the transformer and softeded to a slime, gel or gum on the inside of the coils, dependig on how well it had permeated.. The laminations were really easy to pry loose with a stanleyknife afterward. Only the first layer needed a bit of chiseling with a hammer and screwdriver because of the physical tightness. I was even able to salvage most of the magnet wire in a reusable condition. Apparently the insulation laquer on the wire was indifferent to the drain cleaner. (I wouldn't bet on it, though. So I'm not going to use it for anything high voltage). The whole dissasembly operation was performed in the kitchen sink under gently running water to prevent caustic burns on my fingers.
Warnings about trying to re-use lams

Some of the suggestions are valid, especially meth-chloride, for removing the varnish. Any material that is caustic enough to remove the varnish is surely going to damage the wire and bobbins. I have another warning: If you intend on recycling the lams to build more transformers, don't expect the same performance, in fact, they may not work at all. Laminations receive an oxide-style coating referred to as "core plate, C5, etc." This coating is important as it separates the individual laminations which minimizes the eddy currents, hysterisis losses and core losses. It also minimizes the excitation currents. All of which minimizes the input currents, under full and no-load. Removing the coating may increase the losses to a level that causes the transformers to overheat. As a minimum, you may end up wasting time building a transformer that you can't use.
Here in Argentina there exists a product called "Cloroformo técnico" (Technical chloroform???) and that is buy in the chemical sales in galss bottles, that dissolves very well the varnish, but don´t affect the coil wire nor bobbins. I use it several times to disassemble ferrite transformers from PC monitors and TV and reuse them. About a night it takes to its job, and works pretty fine.
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