A motor or transformer winding is full of tiny gaps: between turns, between layers, and between the coil and its slot insulation. Electrical insulating varnish fills those gaps. It bonds the wires into one solid mass, seals out moisture and dirt, and helps heat escape. Pick the wrong varnish, though, and a good winding can fail early from cracking, poor bonding or a reaction with the materials around it.
This guide explains the main types of electrical insulating varnish and how to choose one for a motor, generator or transformer. If you want to know how varnish itself is produced, see our post on how insulating varnish is made.
What Insulating Varnish Does
- Bonds the winding. Turns and layers are locked together, so the coil resists vibration and the forces of starting and short circuits.
- Seals it. Moisture, dust, oil and chemicals are kept out of the winding.
- Fills air pockets. This improves heat transfer from the copper to the core and reduces partial discharge in higher-voltage machines.
- Adds insulation. It forms an extra layer over the wire enamel and slot insulation.
Varnish is not the same as wire enamel. Enamel is applied to the copper by the wire maker. Impregnating varnish goes on after the coil has been wound and inserted.
Impregnating Varnish vs Finishing Varnish
Impregnating varnish has a low enough viscosity to soak deep into the winding and fill the spaces between wires. It does the bonding and sealing work.
Finishing varnish, also called overcoat or surface varnish, is applied on the outside, usually by spray or brush. It gives a tough, glossy, moisture-resistant skin, and some grades add oil resistance or anti-tracking properties. Many machines get both: an impregnating varnish for the inside and a finishing coat on top.
Solvent-Based vs Solventless Varnish
Solvent-based varnishes carry the resin in a solvent that evaporates during drying and baking. They penetrate well and are forgiving to use. The drawbacks are that escaping solvent leaves small voids, and the fumes need good ventilation.
Solventless varnishes, often called 100% solids resins, use reactive diluents that become part of the cured resin. Almost everything you apply stays in the winding, which gives better void filling and lower emissions. They are the usual choice for vacuum pressure impregnation and trickle impregnation.
Varnishes also differ in how they cure. Air-drying grades cure at room temperature and suit repairs and field work. Most impregnating varnishes are baking grades that need an oven to reach their full properties.
Varnish Types by Resin
Alkyd and Phenolic-Modified
Older general-purpose varnishes, typically Class B. They are still used for low-temperature machines and repair work.
Polyester and Polyesterimide
The modern workhorses. They offer good bond strength and flexibility. Polyester grades are commonly Class F, and polyesterimide grades reach Class F to H.
Epoxy
Epoxy gives excellent bond strength and strong resistance to moisture and chemicals. It is widely used in VPI systems, especially for high-voltage coils, and is typically rated Class F to H.
Silicone
Silicone varnishes are rated Class H and above and stay flexible at high temperatures. They are used in traction motors and other hot-running machines. Their mechanical bond is weaker than epoxy.
Exact thermal ratings depend on the specific product, so always confirm them on the datasheet.
How Varnish Is Applied
- Dip and bake. The wound stator or coil is dipped, drained and baked. It is simple and widely used for small and medium motors, sometimes with two dips.
- Vacuum pressure impregnation (VPI). A vacuum removes trapped air, then pressure forces resin into the winding. It gives the best penetration and is used for medium and high-voltage machines.
- Trickle impregnation. Resin is trickled onto a heated, rotating stator or rotor. It is fast, uses little resin and suits high-volume production.
- Spray or brush. Used mainly for finishing varnish and repairs.
The varnish and the method must suit each other. A varnish made for dip and bake will not necessarily behave well in a VPI tank.
Insulating Varnish Types Compared
| Type | Typical thermal class | Main strengths | Typical use |
| Alkyd or phenolic | B | Easy to use | Repairs, low-temperature motors |
| Polyester | F | Good bond, flexible | General motors, dip and bake |
| Polyesterimide | F to H | High thermal stability | Industrial motors, transformers |
| Epoxy | F to H | High bond strength, chemical resistance | VPI, high-voltage coils |
| Silicone | H and above | Heat resistance, flexibility | Traction and high-temperature machines |
| Solventless (various resins) | Depends on resin | Low emissions, good void filling | VPI, trickle impregnation |
These are typical classes. Individual products vary.
How to Choose the Right Varnish
- Match the thermal class. The varnish should be rated at least as high as the insulation class of the machine.
- Check compatibility. The varnish must work with the wire enamel, slot insulation, sleeves and tapes in the winding. Ask for compatibility data. Absorbent materials such as Nomex 411 and acrylic coated sleeves (see our guide to types of fiberglass sleeve) take varnish well.
- Match your process. Choose a varnish that suits your method, oven temperatures and cure times.
- Consider the environment. Damp, oily or chemical surroundings call for epoxy or a suitable finishing coat. Hot machines that flex need silicone.
- Control viscosity. Keep the varnish at the viscosity the maker specifies, and thin it only with the recommended thinner. The wrong thinner can spoil the cure.
- Watch shelf and tank life. Varnish thickens and ages over time. Follow the storage advice and check viscosity regularly.
Tapes matter too. Our comparison of PVC, glass cloth and polyester insulation tape notes which tapes suit varnished windings.
Final Thoughts
Choose insulating varnish by thermal class first, then by compatibility, application method and environment. Polyester and polyesterimide varnishes cover most dip-and-bake jobs, while solventless epoxy and polyesterimide systems dominate VPI work.
Harnawa supplies Elantas varnishes and thinner from our insulating varnishes and thinner range, along with varnish-friendly materials such as varnished fiberglass sleeving and varnished fiber glass cloth. For how varnish fits into a complete system, see our guide to transformer insulation materials. And if some ends up on your overalls, here is how to remove varnish from clothes. Contact us with your machine class and process to find the right varnish.
Frequently Asked Questions
What is electrical insulating varnish used for?
It is applied to motor, generator and transformer windings to bond the turns together, seal out moisture and dirt, fill air pockets and add a layer of insulation.
What is the difference between solvent-based and solventless varnish?
Solvent-based varnish carries the resin in a solvent that evaporates during curing. Solventless varnish uses reactive diluents that become part of the cured resin, so it fills voids better and gives off fewer emissions.
Which varnish is used for motor rewinding?
Polyester and polyesterimide varnishes are common for dip-and-bake rewinding, while epoxy systems are widely used for VPI. Always match the varnish to the insulation class of the motor.
What is VPI?
Vacuum pressure impregnation. The winding is placed under vacuum to remove air, then resin is forced in under pressure, giving the most complete penetration.
Can I use any thinner with insulating varnish?
No. Use only the thinner recommended by the varnish manufacturer. The wrong thinner can affect curing, bonding and electrical properties.
Is insulating varnish the same as wire enamel?
No. Wire enamel is applied to the copper wire by its manufacturer. Insulating varnish is applied to the finished winding to bond and seal it.
