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