Varnishing my secondary.

Thanks for such an intelligent response

I don't see why not. You should test your varnish on some extra wire and coil form material, to make sure it dried on it, without attacking the wire insulation (swelling, wrinkling, coming lose, etc) or the form material before you risk doing the winding. If you can rig up something like a hand turned lathe and have someone else slowly roll the coil over as you run a brush along it, it should turn out nice and smooth. The first coat (and possibly the only coat) should just saturate the wire, excluding air from under it, if possible. Applying varnish with it turning very slowly lets the varnish soak in, while the air gets pushed out. Once there is varnish all around a turn, the remaining air tends to get trapped. Fast drying varnish is not better than slow drying. It just gives the air less time to escape and you less time to get it smooth. I think I would try a urethane based varnish, first, maybe thinned a bit with mineral spirits (to help it get under the wire). If it dries very thin, you can always add another coat of the full bodied varnish.

Dipping is not a good idea because you wind up with poor penetration (air bubbles) and uneven coverage.

Sure, that's a time-tested method.

Yes, it probably does, and not just to tie the windings down. Air is a Bad Thing to immerse TC secondaries in because it permits corona formation which encourages primary/secondary arcs. Of course this depends on what voltages the thing can produce, but better safe than sorry. Don't forget some way to hold the wire in place while varnishing; if nothing else drill small holes near the ends and tie the wires there, afterward fill the holes with something as similar to the form material as possible.

A slow-drying polyurethane is best (after corona dope, but that's more costly and I don't think a 14-yr-old can buy it as it's basically just clear polystyrene dissolved in toluene) so as to allow any entrapped air to escape before the stuff dries.

DO NOT shake or stir it vigorously before application because that'll encourage air bubbles. Also saturate the brush well for the same reason, and brush strokes should go along the "grain" of the windings (hence the suggested lathe trick). BTW, some say that it's best to paint the form and wind while it's drying followed with a finish coat; you might want to try that with your next coil.

I've never seen polyurethane varnish in "small bottles", but rather in cans (you don't want to look like a dummy at the hardware store, do you?) and a fairly small can should allow you to apply a couple of coats and have enough left over for other projects.

Mark L. Fergerson

In my case yes. Right now my coil is being held on the form with scoth tape, not the best choice.

For a second I thought you said "vanishing my secretary"...

Does it actually *need* varnishing ? It may be a convenient way to ensure it's mechanical stable of course.

Graham

I originally read that as "varnishing my secretary" which seemed a bit kinky.

Regular polyurethene varnish, sprayed or brushed a coat or two, should be fine for tesla coils or administrative assistants.

John

You might want to consider the alternative method described below - but I suggest you discuss it first with the adult who will be responsible for helping you to clear it up if it goes wrong.

If you can get hold of a thickish plastic bag without any holes, you could put the coil into it and add as much varnish as you can spare. Then seal it and by squeezing and tumbling it gently, you could spread the varnish all over the inside and outside of the coil. It may take some time to ensure that every part has been thoroughly soaked, but you are in no hurry because the varnish cannot begin to dry and thicken whilst it is sealed inside the bag.

The tricky part will be removing the coil and hanging it up to drain and dry without getting everything smothered in varnish. Perhaps you can drill a small hole somewhere in it that doesn't matter, then you will be able to hook it out with a loop of stiff wire through the hole. Arrange a nail or hook to hang it from as it dries, with something underneath it to catch the drips.

A serious problem will arise if you have any sharp edges or solder spikes which puncture the bag. As it is a Tesla coil, you ought to have carefully removed those at the outset.

Plan ahead and assume something (perhaps everything) will go wrong. Do the job outdoors, keep an old washing-up bowl or bucket handy to catch spillage, have some rags and white spirit solvent nearby for cleaning up, wear old clothes and shoes.

On Tue, 13 Sep 2005 17:09:01 -0700, John Larkin wroth:

My first impression of "varnishing my secondary" put me in mind of something like "waxing my weasel". Coffee in the keyboard stuff.

Jim

Where's "here"? In AZ USA (and most other parts of the USA) toluene got a bad rep as the solvent in model cement that produces the "buzz" so many kids killed brain cells with. Me, I couldn't stand the odor so I worked with open windows and a fan going.

Yup. I'd be careful with poly foam though. I spec'd clear polystyrene because using the clear solid sprue (from model car parts trees AKA sprues) pretty much guaranteed there were no conductive inclusions, which is important when making your own corona dope. I know this from the days I was transitioning from building model cars to doing electronics, and wanted to put all those saved sprues to good use. Dope made from the sprues that windshields came off had much better holdoff than say black car body parts; the black was probably carbon black.

I understand that nowadays in model kits polystyrene has given way to some kind of resin which probably won't work at all, so perhaps say pure white (no potentially conductive colorants) packing peanuts are a decent alternative source of poly.

Absolutely, and obviously stay the hell away from open flames, small kids, pets, etc.

Mark L. Fergerson

Here, I suspect even a 14-yo can buy toluene at a tiling suppliers, as it's commonly used to clean up tile cement. Put some in a big jar and feed as much polystyrene as you see fit - it's amazing how much polystyrene foam you can dissolve into a given jar. If you can't get toluene, kerosene works just as well, but will take longer to dry.

In either case, make sure you are outdoors in a drafty area with nothing electric operating nearby!

You and Ross Merchant

Good Luck! Rich

Alternatively. It is possible to simply immerse the secondary in oil.

Australia. I suspect toluene has been removed from model aircraft dope here also, the only place I know it's used is in tiling.

If it's plain white, I doubt there's anything conductive in it. I wouldn't go with peanuts, just break up the moldings from your last monitor, TV or other household appliance. Certainly it seems to generate enough static electricity.

Watch out for the ones made out of edible(ish) corn products. They don't contain much polystyrene. If they dissolve in water, they're not polystyrene :)

Or if they look just like popcorn... (got a computer packed in popcorn once. Mother started eating it.)