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Too late now, but I try to give these older, used at higher voltage motors, a good dose of spray-on, good quality insulating paint. Apply at different positions so it runs down the slots, lower windings, and, if possible, apply multiple coats while the arm. is slowly rotating on something like a BBQ rotisserie.

Motor shops can also do a commercial dip, rotate, bake, and re-balance. Sometimes they use a varnish, sometimes an epoxy for the severest duty. One trade-off is that the extra insulation may trap more heat in the windings - something to watch out for.
 

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QUESTION
Dip or laqueur the armature? - I'm inclined to leave it the way that Hitachi left it
Yes, but Hitachi probably dipped it many decades ago. The varnish may have dried out, cracked, or been worn away by now. As well as a insulator, the varnish glues the windings in place to keep them from shifting around and chafing the insulation. That's probably what doomed your old motor.

Check what the motor shops suggest for a high voltage, current, and RPM severe duty usage. Also, check the condition of the fiberglass (usually) tension bands around the exposed windings. The glue holding them on may have dried out and weakened.
 

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Duncan! You're running the motor at 8-10 times the stock voltage. The motor is going to need all the help it can get to survive. At least apply the good part of a rattle can(spray paint can) of a good insulating varnish(paint) designed for motor windings from an industrial supplier. It's cheap insurance for the kind of thrashing you're going to put this motor through.

Since the paint is solvent based, the heavy coatings needed will take some time to dry-several days or more. Also, it can be baked. I've used it a lot. If the armature is rotated slowly mechanically or by hand incrementally every 10 to 20 minutes, initially, I've gotten good coverage and never had an imbalance problem. And, it makes the windings look good, easier to clean, shed dirt, and impress your fellow motor heads!!
 

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Good points about the cleaning before painting. I remember one hydro dam generator was cleaned with a walnut shell blast medium before painting. I have successfully used Stoddard (close to mineral spirits, paint thinner) solvent to clean the windings on dozens of motors from small universal motors in power tools to motors larger than yours. I've never seen these mild solvents harm the insulation coatings on motor windings. If you not sure, test the solvent on a sample area. Make sure the solvent is completely dried out before painting.

One large wound-rotor induction motor from a large printing press was so heavily caked with paper dust and grease, it needed to be scrubbed with soap and a water blast. After it was cleaned, it was rinsed with a lot of distilled water and baked dry. After it was dried, and before painting, it megaohmed out as good.

Did you have a chance to talk to a motor shop about their recommendations?
 

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As far as the paint adhering to the surfaces, the cloth sleeves(if used) around the wires, the insulating paper, and the wood wedges(if used) that lock the windings in the slots all soak up the paint somewhat to increase the insulation around the wires. And, as you say, "glue" the parts together to keep them from shifting around and chafing.

I've also damaged windings . Sometimes, not always, you can carefully pry back the wires(if they're not burned-up) in the damaged area, insert pieces of the insulating paper to re-insulate the wires, and repaint. Motor shops will give or sell you the small amounts of paper as well as the insulating sleeve material, if needed. They also would be a good source for the insulating paint.

You're right. It is a chore to do this. And, it sounds like you can't wait to get back on the road!
 
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