If you wind your own coils whether they are for an electric motor, electromagnet, radio coil or tesla coil or whatever, when you've wound it and have it the way you want it if you paint it with coil dope it will hold it's shape and won't hum in use.
If you're winding an rf coil for a radio receiver or transmitter then it's really important to dope your coil. An rf coil that has been tuned will change it's characteristics if the windings move. When this happens your radio is not aligned properly anymore.
You can still buy coil dope but it's expensive and it's really easy and inexpensive to make your own.
All you need are a can of Toluene from the paint store or hardware store, a clean glass jar that has an airtight lid and some white packing peanuts. Make that a bunch of packing peanuts. You will be amazed and how many will dissolve in that little bitty glass jar of solvent.
The peanuts have to be white and they have to be made of polystyrene. The ones made in the shape of an "S" work best. The ones that are a figure "8" don't work. There are some of the white ones that are made of starch that won't work. If they dissolve in water don't use them. I've also used the white portions of soft drink cups but they didn't make what I would call "high quality stuff". The green or pink kind don't dissolve easily and leave a residue that is probably the material added as coloring.
Step 1: Here's a Coil That Might Need Doping.
I didn't dope this coil but if it's not wound tight enough running the contactor back and forth might cause the turns to bunch up. Maybe I'll coat the coil after I've made my dope.
Step 2: How to Start
You must do this in a well ventilated space. Even when you use your coil dope you need to be in a well ventilated space. Toluene is nasty stuff in a closed room.
Pour your jar about half full of toluene. The close the can tight and set it aside.
Drop a few of the peanuts in and if they dissolve you have the right kind. They should dissolve almost as soon as they hit the liquid. Drop more and more in the liquid. You'll probably need about 2-4 gallons of peanuts to get your dope to the correct consistency. You want it to be about as thick as pancake syrup. If you get it too thick (not likely) you can add a very slight amount of toluene.
What you'll end up with is a slightly thick, milky liquid that you can paint all over your coil or if it's small you can just dip it in. It takes about 30 minutes for it to dry.
Keep the jar tightly covered when not in use.
If your dope starts thickening in the jar add very small amounts of toluene to thin it again. If it goes solid again it can be redissolved by adding toluene.
I've seen electronic tests comparing homemade dope against store bought dope and electrically they were about the same.
Remember to use with plenty of ventilation and away from flames. This stuff is very flammable.