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.