Which winding varnish should you use?
Choose the right varnish in 4 steps by thermal class, oven availability and motor type.
1. Determine the thermal class
The first and most important rule: the thermal class of the varnish must not be lower than that of the winding (the enameled wire). A lower-class varnish softens, cracks or carbonizes as the motor heats up, and the winding fails early. A higher-class varnish can be used on a lower-class winding without problems.
| Class | Max. temperature | Typical use |
|---|---|---|
| B | 130 °C | General-purpose small and medium motors |
| F | 155 °C | Industrial motors and pumps; the most common class |
| H | 180 °C | Heavy duty, high temperature, dry-type transformers |
You can find the class on the motor nameplate or on the enameled wire packaging. If in doubt, treating the motor as class F and using a class F varnish is the safe choice most workshops make.
2. Do you have an oven? Air-drying or baking?
| Air-drying (OR 1xx) | Baking (OR 3xx) | |
|---|---|---|
| Curing | At room temperature | In an oven, following the recommended temperature–time schedule |
| Equipment | No oven needed | Curing oven required |
| Bond strength and durability | Good | Highest; resistant to hot transformer oil |
| Best for | Repair shops, field work, single motors | Series winding, factories, heavy duty |
In short: without an oven, use the air-drying OR 1xx series; with an oven and a motor for heavy duty, use the baking OR 3xx series. For a detailed comparison, see our guide on air-drying vs baking varnish.
3. Choose by motor and job
| Job | Recommended Orchem product |
|---|---|
| General motor repair, field work | Air-drying winding varnish (OR 1xx) |
| Series winding, factory production | Baking winding varnish (OR 3xx) |
| Armatures, rotors, high-speed motors | Armature varnish |
| Submersible pumps and humid environments | Baking OR 3xx; without an oven, OR 1xx followed by an insulating top coat |
| Low and medium voltage transformers | Transformer insulating varnish |
| Variac outer surface | Variac varnish |
| Touch-ups, connections and terminals | Motor insulating spray |
4. Consider the application method
- Dipping: gives the best penetration throughout the winding. Keep the winding immersed until bubbles stop rising, then let it drain.
- Brush: practical for single motors and repairs. Work the varnish well into the winding.
- Spray gun / aerosol: suitable for top coats, touch-ups and connections.
Getting the right consistency matters as much as choosing the right varnish. Our thinner guide explains which thinner to use with each varnish and at what ratio.
Common mistakes
- Using a varnish of too low a class: class B varnish on a class F winding means early failure once the motor heats up.
- Leaving baking varnish to air-dry: the surface may look dry, but it does not cure fully; it stays tacky and does not provide the expected strength.
- Varnishing a damp or oily winding: the varnish does not adhere and moisture is trapped inside. Clean and preheat old motors first.
- Using the wrong thinner: the varnish can separate, and drying and insulation can be impaired.
- Not draining excess varnish: pooled varnish dries slowly and leaves a thick, soft layer.
Questions
Can the same winding varnish be used for every motor?
No. The varnish should be chosen according to the motor's thermal class, whether you have an oven, and the type of job (general motor, armature, transformer, submersible pump).
Can class F varnish be used on a class B motor?
Yes. A higher-class varnish can be used on a lower-class winding without problems. The reverse — class B varnish on a class F winding — should be avoided.
Does baking varnish dry in air?
No. Baking varnish cures fully only when baked at the recommended temperature and time. Left in air, it stays tacky and does not deliver the expected durability.
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