# Resistor coating flaking off of a Parma Resistor



## slotcar58 (Jun 21, 2007)

Does anyone know where to get a bottle of resistor coating for Parma resistors? I have tried to purchase this from Parma and they do not sell bottles of the coating. I have 3 or 4 contollers where the coating is falking off. Thanks in advance for your help.


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## twolff (May 11, 2007)

It's been falling off mine since they were new too. I wouldn't bother replacing it with the same material, it's prob. just going to flake off again. I've been thinking of using epoxy on mine, but I know that epoxy will soften with heat.


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## NTxSlotCars (May 27, 2008)

One of our guys uses clear nail polish, says that works good. I haven't tried it yet.


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## 22tall (Jul 28, 2002)

I would like an answer to this one too. That orange stuff is way too brittle.


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## mahorsc (Mar 26, 2008)

we have always used epoxy after dry sand flat to uncover wire 
make controler super smooth


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## Illinislotfan (Mar 8, 2009)

I was going to suggest something like liquid electrical tape, then did a quick google search, and found this. I don't know anything about parma controllers, I'm still trying to master the Aurora thumb controller, but maybe this will help someone out.

http://slotcarillustrated.com/portal/forums/showthread.php?p=278761


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## twolff (May 11, 2007)

JB Weld might do the trick. I've got some finishing epoxy that is normally used for fiberglass and other composit structures that I'll brush on the whole resistor next time I have one apart.


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## CaptnAndy (Mar 27, 2009)

*Controller Resistor Coatings*

Most power rheostats have a ceramic coating, which is painted & then fused in a high temperature oven. Flaking is usually caused by the nichrome resistance wire overheating, expanding, & fracturing the coating. 

If you are running too large a resistance for the motor & track, & spend most of the time with the wiper in the center, it will overheat. It will also overheat if you hook up the controller wrong. 

Most paints & epoxies will not take high temperatures without degrading. A ceramic coating will likely chip again, if the same type of overheating occurs.

My advice, is to not attempt to re-coat the existing resistor. Run it to destruction, & then replace it, possibly with a lower resistance resistor. The Parma Turbo controller has a resistor with a higher power rating, & it has a much larger heat sink. If you are replacing a Turbo resistor use the Parma 311G, which has a better coating. 

If you are considering moving up from the Sebring Controller, take a look at the Professor Motor Electronic Controllers. They are an excellent value for many applications.

The other factor is driving style, increasing the time that the controller is at the full speed or brake position will reduce the dissipation in the controller.


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## mahorsc (Mar 26, 2008)

i have a 60 ohm parma i did i the 90s with jb weld and 125 ohm i did the same that has 100s of hrs on both and had no trouble


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## twolff (May 11, 2007)

I'lll have to give the JB Weld a try. Nothing would be better than the dayglo stuff Parma uses on the Econos because that would mean nothing would be flaking off and making a mess on the track. I took a peek at them yesterday and they all appear to be very close to being done "shedding".


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## AfxToo (Aug 29, 2003)

> The other factor is driving style, increasing the time that the controller is at the full speed or brake position will reduce the dissipation in the controller.


This is absolutely correct, as is the posting about using too high of a resistance controller. The inherent problem with resistor controllers, which are current throttling devices, is that all power that does not go to the car must be dissipated in the controller. The worst thing you can do is to hold a resistor based controller at one setting (other than full on). Warnings are included with these controllers that state not to do this.

The resistors used in the Econo controllers are not really coated. There is a little flaky material there just to physically stabilize the windings. Other than that it does not do a whole lot.

The best coating material to use on these would an electrical grade epoxy or urethane *potting compound* designed specifically for this type of electrical application. You'd want to use a compound that has high thermal conductivity and high resistance to thermal shock.The 3M DP2070 or Loctite 3860 products would work very well. McMaster-Carr sells a variety of potting compounds (search for "potting compound") and lists the properties of each type. Be careful about "heat transfer epoxies" because some of them are electrically conductive, which would be a bad thing.Room temperature curing is also a must.

These potting compounds are designed for completely encapsulating electrical and electronic components, and all you really want to do here is to replace the fairly thin strip of missing flaky material with a strip of potting epoxy or urethane. You would have to mask off the rest of the resistor to prevent the potting material from getting on it and first remove all of the existing loose material. The last thing you want to do is to encapsulate the loose coating in an epoxy shell.

The problem here is in finding the right material in sufficiently small quantities to make it worth your while. If you have a bunch of controllers you want to fix up, then spending $10-$15 dollars for proper potting material may be worth your investment. If you only want to fix one or two resistors, then just about any non electrically conductive epoxy applied as a winding stabilization strip, without coating the whole backside of the resistor, would be good enough. Just make sure the service temperature is high, 250 degrees Fahrenheit or higher, like Loctite 1166731.


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## NTxSlotCars (May 27, 2008)

How about Miracle Grow? or QuickCrete?


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## Hornet (Dec 1, 2005)

Back in the early days the common fix was to wire wheel the orginal coating off,then heat the resistor in an oven ,then let JB Weld flow over the warmed up resistor from the backside.
Make yourself a little stand from coat hanger wire that'll hold the resistor face down in your oven ,and while you coat it.When applying the JB Weld try not to get to much on the face of the resistor.Once the JB has cured,lightly sand the face of the resistor with a fairly fine grade of sandpaper,stick it back in your controller and you'll be good to go for years.
Yes JB Weld conducts electricity,but not enough to ever effect the operation of your low voltage/low current draw controller circuit


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## Jim Norton (Jun 29, 2007)

*Oil?*

I just put a drop of oil on the resistor and that seems to make it smooth again. When it goes away....repeat.

Jim Norton
Huntsville, Alabama


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## twolff (May 11, 2007)

Took a couple of my controllers apart this evening. Nothing holds the wire in place on the resistor except the dayglow stuff on the backside and the spot weld at each end. I hit the front of one of the resistors with some JB Weld and heated it with a heat gun to make it flow better. The other resistor came apart when I was removing it from the frame. The windings loosened up and more or less fell off. It's history. I'll leave the one that survived to cure a couple days and see how it sands out.

All of my econo resistors will be getting a coat of something to hold the winding in place once I determine what I like the most.


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