View Full Version : Insulating Varnish & transformers

03-12-2006, 02:49 PM
rewound a transformer and now need to varnish the windings. I think what i need is insulating varnish. do you guys know if this is something very different from regular varnish and what type of store one obtains it from, ie motor shop, etc? last question, do i have to be concerned about it acting as a solvent on the magnet wire and creating a short? thanks in advnace

03-12-2006, 03:34 PM
Glyptal is usually the preferred choice for electrical applications.Hope these links help.


03-12-2006, 04:20 PM
I would stay away from the Glyptal/air-dried coatings for windings. Down within the windings where they do not get exposed to air/oxygen the varnish will remain a gooey mess. Yes, I know air dried coatings have been used for this application, and this leads to failure down the road.

I would recommend a room temp. cure 2-part unsaturated polyester as used in body putty. It should be thin enough to penetrate the windings. The gel time can be regulated by the amount of hardener used. A 2-part epoxy could also be used if the viscosity is appropriate. The windings can also be heated which increases the penetration of the varnish and will decrease the cure time.

MSC and Mcmaster sell these types of products.

If you need more info, give a holler.


03-12-2006, 05:16 PM
Insulating varnish is similar to regular varnish but the insulating varnish has a higher heat resistance.

Use Glyptal. It will air cure or you can heat cure it. Preheat to about 100-120 in the oven. Brush it on heavy and allow to set/soak a few minutes then apply another coat. If the winding keeps wicking up the Glyptal, keep applying coats until it stops. The moderate heating of the winding will thin the varnish and allow it to wick between the windings.

When it's taken all it will take, let it air cure (and drip) for about 15-20 minutes. If you want to heat cure it and speed handling, bake at about 140-150 for 2 hours. Otherwise, it will take overnight to air cure.

This is the best price I've found for Glyptal.


Forrest Addy
03-12-2006, 05:40 PM
The motor and transformer industry have developed over the years lasting and effective ways of immobilizing windings and that is a heat cured varnish or resin. Air dry insulating materials are not used escept for surface application.

Take the assembled transformer to a motor shop and have them dip and bake it for you. It won't cost much and it ensures the windings are immobilized against relative movement under the influence of alternating current magnetic fields. There is nothing like an overnight dip in the real goods to ensure saturation.

Because transformers are layer wound, there are few interstices for the impregnating varnish to insinuate itself into the conductors. Its common practice for each layer to get a few strokes from a varnish brush during manufacture. This ensures at least token bond between conductors and support.

[This message has been edited by Forrest Addy (edited 03-12-2006).]

03-12-2006, 07:40 PM
Forrest is right about having someone else do it for the best application. I often forget that some of you live in/near a large city and have the advantage of specialized suppliers at your finger tips. http://bbs.homeshopmachinist.net//wink.gif

You didn't mention the size of the transformer or it's winding size. If it's the size of your fist or smaller, do it yourself. The last time I went to a motor shop, it cost me $50 to get into the door. I hope your suppliers are more cost effective for you.

03-12-2006, 07:46 PM
thanks guys, as always this place is a font of knowledge.

Forrest, I had a grin when i read your line about few interstices, says me to myself, he hasn't seen my windings! I'm getting the voltage I want it the new secondary is NOT the nice neat winding that the primary is...no doubt it was wound on a bobbin before the E was welded shut. Even with a extra set of hands helping i was not able to get a very neat job of the windings. I'm think that there are so many cavities with the poor winding job that maybe the dip or polyester is the way to go.

thanks again for all the ideas and info

ACF - any other hints as to how to find that kind of polyester? is it basically Gelcoat? i'm not having much luck googling. epoxy also sounds interesting, would i matter much what epoxy was used? iirc you control viscosity by heat, so I could apply several thin applications to get in the spaces

good point CCW, the transformer is big, pulled it from a microwave, rewound the secondaries to give me about 28 rms and 8 rms, higher voltage will be at max 6 or 7 amps driving stepper motors, low is neglible amps, fans, ttl etc.

I'm in Toronot, there are motor shops around, but if i start spending $50 on it, i should have just bought a transformer (yeah i know, don't say it...trying to make this as much diy as i can)

[This message has been edited by Mcgyver (edited 03-12-2006).]

J Tiers
03-12-2006, 07:51 PM
They are commonly vacuum-dipped, to get varnish everywhere. We specify that.

The heat-cured is the best type, although others do work. The varnishes used are intended for the application, and differ by heat rating, and heat class of transformer.

Glyptal is best used for exposed areas only, it doesn't soak-in well, although its OK as a varnish. You CAN pull it in with a vacuum, but I don't actually know about its heat resistance.....

Get it done by a motor shop if you can, that will be best.

The correct type varnish will not affect the winding insulation at all. But you are right to be concerned, insulating systems vary, and not everything is compatible.

Any transformer will have used a UL insulation system, and everything will carry UL recognitions and have a file number.

No UL file number? I wouldn't advise using it in a power transformer.

[This message has been edited by J Tiers (edited 03-12-2006).]

03-12-2006, 11:36 PM
Back in the good old days (50s & 60s)I hand wound a few small motors and transformers. For lack of anything better I used regular orange shellac. (Some of the old books back then actually recommended it.) They ran for years with no problems. I air dried and then put them in a warm oven. No warranty on this. Remember that the solvent in shellac is alcohol and that it is flammable


03-13-2006, 08:56 AM

The gelcoat type of of resin is what I suggested.

Check out www.mscdirect.com (http://www.mscdirect.com) and look at part number 00001958.

A commercial shop that can bake a specialized coating would be ideal, as baked coatings are just about always better than room temp. cured products as others have mentioned.

If you don't mind me asking, what type of apparatus are you having VPI'ed (vacuum pressure impregnation)? The VPI process is the best way to go as it fills and penetrates even coils with layers of tape. You're probably using a P.D. George resin system as they are just about the only big insulating resin supplier left in the country.


03-13-2006, 08:05 PM
CCWKen - I am in San Antonio - what motor shop did you use? I need to have a couple of motors cleaned and dipped (sounds like taking a dog to the vet). One is a Taiwanese motor off an Enco bench knee mill that has been stored for 20 years, the other is off a Sheldon lathe that I am slowly restoring. Thanks! A.T.

03-13-2006, 11:01 PM
I rewound a clutch magnet coil on a riding mower,used nothing more than Minwax polyuerathane and it has worked for five years now,cost $3.85 for a pint can.

I did brush mine on layer over layer during winding,but it is thin enough to saturate.

03-13-2006, 11:44 PM
Hey AT, Howdy neighbor! I can't remember the name at the moment (it's been 2-3 years) but it was on Basse Rd. between West Ave. and San Pedro. North side of the street.

03-14-2006, 12:39 AM
If you shop around for your winding wire, you can find wire that is precoated with an epoxy that will harden after wound and heated. It seems like a great advancement to me.

03-14-2006, 11:16 AM
I rewound the fields on a 2000 watt Generac under hood alternator that puts out 117 vac. I used red spray on insulating varnish available in a rattle can and let it dry for a week. That was maybe 10 years ago. It still works. I also cleaned the windings on my Strands drill press with an ethanol rinse when I overhauled it and used the same insulating varnish on them. It's working fine too.

J Tiers
03-14-2006, 12:46 PM
<font face="Verdana, Arial" size="2">Originally posted by Evan:
I rewound the fields on a 2000 watt Generac under hood alternator that puts out 117 vac. I used red spray on insulating varnish available in a rattle can and let it dry for a week. </font>

I've done that, and worse, myself..... But it's another thing to "recommend" doing it......

The ones we specify it on are SMPS transformers in the 250W to 3kW area. Regular transformer vendors for "iron" transformers do it as a matter of course, to prevent vibration.

Chinese motor manufacturers seem not to do it much at all....

03-14-2006, 12:55 PM

It's the correct varnish for the job and soaks in very well. I'm not talking about just giving a light spray to the surface of the windings but really soaking them through. I dried them in the hot summer sun and if that isn't available a slow low temp bake will do well.

03-14-2006, 03:28 PM
<font face="Verdana, Arial" size="2">Originally posted by Evan:

It's the correct varnish for the job and soaks in very well. I'm not talking about just giving a light spray to the surface of the windings but really soaking them through. I dried them in the hot summer sun and if that isn't available a slow low temp bake will do well.</font>

While the air dry coating may have worked in your application, it was definitely not the correct product to use. Your rewind job may have worked without any varnish applied to it. These air dry coatings will only cure by oxidation. Even baking them without exposure to oxygen will not cure them.

One of the most important properties of the varnish is to bond adjacent wires together along with any other insulating compounds in the unit, such as nomex papers and tie wires. When the varnish is not cured and is a gummy mass the bond strength in just about non-existent. Years down the road when the varnish may have seen enough exposure to oxygen it will start to bond everything together. If the unit can last that long you'll be OK, if not you'll have a failure.

If you sprayed this on and did not penetrate down deep in the windings then the outside would cure were exposed to the oxygen and the inside would have no varnish to cure anyway. Most varnish applications are dipped and baked, VPI'ed, or trickle coated to afford deep penetration of the varnish.

The unsatuated polyester system will cure without heat and without air and assuming you get good penetration will be far superior to any air dry system.

So you're VPI'ing to bond the laminations together. I see.


03-14-2006, 11:03 PM
<font face="Verdana, Arial" size="2">These air dry coatings will only cure by oxidation. Even baking them without exposure to oxygen will not cure them.</font>

Glyptal is a alkyd polymer. An alkyd enamel paint/varnish so to speak. There's enough exposure to moisture in the air ("oxygen") on each coat to start the reaction of curing. Baking takes the place of "oxygen" in the reaction by supplying heat that speeds the polymer cross-linking and thus hardening. You only need one or the other, not both. Heat is just faster.

Have you ever opened a can of alkyd enamel paint, used a little and put it back on the shelf? When you went back months or years later and opened the can, there was a skin over the paint. That top layer became a polymer and there's nothing you can do to redissolve it into the paint under it. That skin is probably 100-200 mills thick. Only the top layer of molecules were exposed to "oxygen" as you call it. It's not the air, it's the moisture in the air or heat that starts the reaction.

That's also why auto painters use heat to cure paint. It's not necessary, it just speeds the curing. It has nothing to do with "oxygen" other than the water molecule--H2O.

[This message has been edited by CCWKen (edited 03-14-2006).]

03-15-2006, 07:53 AM

Alkyd's are all based on vegetable based drying oils, and therefore all reguire oxygen to cure, moisture has nothing do due with the crosslinking reaction. Heating in the presence of Oxygen defineitly will speed the cure and also give you a better coating, but moisture has no part in the curing of an alkyd.

There are coatings that require moistue to cure, but these are based on isocyanates, and generally drying oils are not used in these coatings. There are free isocyanate groups attached to the polymer that react with the moisture to crosslink the coating.


03-15-2006, 09:00 AM
Modified alkyd resins may depend on oxygen to cure or may be crosslinked by temperature alone depending on formulation.

<font face="Verdana, Arial" size="2">
Alkyd resins are the reaction product of an oil or fatty acid, polyol(s) and polyacids. These polymers are supplied in solvents and can cure by reaction with oxygen or amino based crosslinking resins to form tough, durable films. The choice and amount of oil in the polymer determines the dry rate and solubility of the polymer in aliphatic solvents. Long (over 60 %) and medium (40-60%) oil alkyds are supplied in low odor aliphatic solvents and are suitable for architectural and maintenance finishes. Short (under 40%) oil alkyds are supplied in aromatic solvents and are used in fast air dry and bake finishes.


The insulating varnish type I use is produced by MG Chemicals in Vancouver and is a short air or bake dry finish modified alkyd that meets ASTM-D-149 specification. It may be air dryed or baked to crosslink. The baking temperature is about 150 to 200F.

In most cases any uncured resin trapped in the windings will be cured the first time the item is used as the temperature rise under use will usually be sufficient to harden the product. The solvent used in the product I use is Xylene which is extremely volatile and easily evaporated at elevated temperature.

03-15-2006, 06:03 PM

Unmodified alkyd resins will not cure without oxygen. They may be cured at room temp. or they can be cured at elevated temp.(with oxygen present). The heat cure will yield a better film. This is true, I wouldn't bulls**t you.

Try this test.
Take some unmodified alkyd resin and place it in a dish or cup so it is maybe a .25 inch thick and place it in an oven at 300 F. for a few hours. The solvent will evaporate and the surface of the remaining resin will be dry but underneath it will still be uncured and liquid, possibly a high viscosity liquid, but still uncured. You can see this very easy when the resin is still hot. You see the same phenomena in the can after you open it, use some, and close it back up. The top skins over due to the oxygen reaction, but underneath you still have good paint.

I've formulated and cooked thousands of alkyd resins when I was working and they are made by "cooking" them at about 230 C.and they do not cure in the kettle at this temp. The reason they don't cure/gel at these temps. is because the reaction is run under nitrogen to exclude the presence of oxygen.If you let enough oxygen into the kettle you'll have a gelled mess (nasty cleanup job then). If heat alone would gel these alkyds there would be no way to make them.

The statement about the first time you heat it under use "will usually be sufficient to harden the product" was written by a salesman to sell his product and is B.S.

Alkyds are very low cost when compared to the varnishes used in the electrical industry and virtually no one uses them because they will not do the job. The mainstay of the industry before the EPA crap was alkyd resins coreacted with phenolics and all were bake coatings. These cure down deep in the windings because the phenolic resin actually does the crosslinking.

Run the test if you don't believe me and see what happens.

I come to this forum to learn about what I don't know about machining. Macgyvers question gives me a chance to contribute something I do know about.


03-15-2006, 07:22 PM
There are many modified anerobic alkyd resins. They are widely used in the printing industry which is where I have first encountered such resins. They are used in the toners of color copiers which I have decades of experience with. I assure you, thermosetting alkyd paints and varnish do exist that require no oxygen to promote the reaction. Some of the heat setting alkyds contain materials similar to epoxies and are catalyzed by heat which promotes the crosslink reaction.

This is the reaction for heat catalyzed melamine modified alkyd resin which is commonly used in baking enamels. The oxygen in the reaction is found in the resins themselves and no outside source is required.


For more information see here:


03-15-2006, 11:16 PM
You have now added another resin to your alkyd that changes the entire ballgame. We were talking of unmodified alkyds now we're talking about modifying them with urea/melamine formaldehyde resins, make up your mind because these are all bake coatings. This would be similar to the phenolic modified system I previously talked about. If you air dry/ no bake this system it will also be a gummy mass inside and low temp baking (around 200 F.) will not be enough heat to set about crosslinking. These systems are what are used in the electrical industry where an alkyd is crosslinkied with an aminoplast resin at a bake temp. of usually 135C. or higher. You don't buy these in a spray can at your local store.

Unmodified alkyd resins will not cure without the presence of Oxygen, heat or no heat. This is why aminoplast resins were added to the alkyd, but these require a substantial bake to afford crosslinking.

There is NO OXYGEN GENERATED by an aminoplast resin and none needed for the aminoplast to crosslink with the alkyd, but there will still be no reaction of the double bonds of the alkyd without oxygen. However if the bake temp is high enough crosslinking will occur through the aminoplast resin, but without oxygen the double bonds of the alkyd will remain unreacted.

Why don't you run the test previously mentioned and see what happens? If it is an unmodified alkyd you'll end up with a skin over top of liquid, if it has aminoplast resin in it it will cure completely through.

I'm sorry I went this far Evan, cause it looks like you're just bustin my balls so to speak.


03-16-2006, 01:52 AM
If you look at my posts I never mentioned unmodified alkyd resin at all. The product I refered to (and use) and that JT took exception to is a modified alkyd resin product that is heat crosslinked as well as air dry.

I didn't bring up unmodified alkyd resin and wasn't talking about them. The subject of unmodified alkyd resin needing oxygen to cure has nothing to do with the suggestion I made.

03-16-2006, 08:38 PM
I have used the MG red insulating varnish and find it to be a good product and certainly is thick enough to hide your winding blemishes. Check out their website for spec's. Do not buy the small bottles unless you open the package at the distributor. I did not do this and got home, opened the bottle and it was dried out. The 1 litre can is about 37.00 cdn. I'm happy with the job it gave me. I redid the transformers on my welder.

J Tiers
03-16-2006, 11:49 PM

As far as why we VPI, it is mostly to keep the windings bound together in a lump. On LF transformers, so they don't buzz, (lams too, of course).

On HF transformers, so everything stays stable where it was on test. With those, exactly where and how the wire lays can make a considerable difference in performance.

I have seen cases where the winding wasn't spread out enough, and the output voltage changed 50% (it was a feedback winding, and the leakage inductance spikes fooled the feedback).

Also to somewhat improve the cooling and heat transfer. Varnish isn't a great heat conductor, but its better than air.

If Evan used a crosslinking varnish, it may have been OK if he could get it to kick off.

My "worse" referred to using Ace Hardware varnish a time or two for one-off stuff for tests.

I suspect the chinese of doing that also, some new transformers kinda stink of something very like spar varnish when fresh out of the box. After a while they don't smell anymore.