How to successfully coat a secondary with epoxy resin

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Scott Hanson by way of Terry Fritz <teslalist-at-qwest-dot-net>" <huil888-at-surfside-dot-net>

Obviously, several writers who have tried epoxy coatings for their 
secondaries have encountered difficulty, and concluded that epoxy is 
worthless or just too hard to do right.

I use nothing but epoxy, and consistently obtain great results, but it 
requires a bit more attention to detail than simply brushing on a coat of 
polyurethane.

In the end, it depends what you want from a coating. At the minimum, you 
want something that will bond the windings to the form. Any solvent-based 
polyurethane varnish will achieve this. But, even with multiple 
applications you will not have a coating that provides much electrical 
insulation or physical protection from bumps and scrapes. I started out 
using commercial spray-on high temperature varnish designed for 
impregnating motor windings. This took a very long time to dry, and each 
subsequent coat took longer and longer to dry to a tack-free surface. Even 
many coats resulted in a rather thin coating, compared to what can be 
achieved from a single coat of epoxy. Additionally, the total "wet time" of 
multiple coats greatly increases the total time that the finish can pick up 
dust, etc. I also tried several different brands of solvent-based 
polyurethane varnish, with results very similar to those obtained with the 
motor varnish.

At the other extreme is a heavy (~.050" +) coating of clear epoxy. I use 
"Enviro-Tex Lite" by Environmental Technologies, a clear epoxy resin finish 
designed for coating bar-tops, tables, etc. Its distributed by Ace/True 
Value Hardware, although not all stores stock it. (There are many other 
clear two-part epoxies that can be used, but this is available in pint and 
quart kits, where most of the industrial epoxys are sold in a minimum 
two-gallon kit.).  This material provides a very thick, impact and scratch 
resistant coating that makes the windings appears as if the coil is encased 
in a glass sleeve. Additionally, I am firmly convinced that this heavy an 
insulation greatly helps to protect a coil from flashover or insulation 
breakdown, even when very high coupling coefficients are used. I use epoxy 
coatings exclusively, and have never had a secondary damaged from external 
flashover or insulation breakdown (although I did have one fail from 
internal flashover and arc-tracking, before I started using internal baffles).

Epoxy coatings absolutely require a low-speed winding jig to keep the form 
rotating while the resin cures. Very thin coats of polyurethane can be 
sprayed on a coil without a winder, but heavy coats will inevitably "sag" 
unless the form is kept rotating.

Here is my epoxy coating process, developed over several years and dozens 
of secondaries:

1. Set the winder up in as dust-free a location as possible. I use a 
bathroom that's just been wet-mopped, with all doors and windows closed. 
Make sure that the ambient temperature is within the range recommended by 
the resin manufacturer (usually 68 - 78 degrees F).

2. Very carefully wipe down the coil with a dust-free cloth barely 
moistened with isopropyl alcohol. (If there are any traces of  oil, 
fingerprints, etc on the wire, it will cause "fish-eyes" in the 
coating.)  After this has completely evaporated, perform a final wipe with 
a furniture maker's "tack cloth" to remove the last tiny specs of dust.

3. Determine the amount of resin you will need to mix. Calculate the 
surface area of the coil, and calculate the volume of coating required 
based on a coating thickness of around .050", plus a few tablespoons extra. 
Its far better to have a little bit of resin left over than to try to 
hurriedly mix up a few tablespoons more if you run short.

4. Carefully measure out EXACTLY equal parts of resin and hardener. Make 
sure to use an absolutely clean glass or disposable plastic mixing 
container, and use a clean plastic rod or wooden coffee stirrer to mix. Any 
contamination from the mixing container or stirring rod can cause a disaster.

5. You MUST MIX THE TWO PARTS VERY, VERY thoroughly. Because both 
components are clear, it is difficult to verify when mixing is complete. 
I'd recommend at least 3 or 4 minutes of  continuous vigorous mixing, 
depending on the total volume to be mixed. As the pot life of mixed resin 
is at least 45 minutes to an hour, take the extra time to ensure complete 
mixing.

3. If you've got the means, vacuum de-gas the mixture to remove the 
bubbles. If you can't de-gas, you can still get a bubble-free coating, but 
it will take a bit more effort.

4. Turn on the winder and adjust the speed to about 1 revolution every 15 
to 20 seconds, depending on the diameter of the coil. Too high a speed will 
result in slightly raised circumferential rings of resin that won't become 
visible for an hour or two.

5. Hold a 3" wide polyurethane foam paintbrush at 12 o"clock position above 
the coil, a few inches from one end of the coil, and begin to pour a tiny 
stream of resin in front of the brush. Allow just the very tip of the brush 
to contact the coil with very low pressure. The first revolution or two 
will only start to spread the resin, but each revolution will spread the 
resin into a wider and wider stripe. Slowly move the brush axially along 
the coil as you continue to pour a very small stream of resin in front of 
the brush. Eventually you will have a "barber-pole" appearing stripe of 
resin from end to end of the coil. Now, reverse the brush motion and go 
back in the opposite direction, not adding any more resin but just 
spreading the existing resin stripe wider and wider. Remember to just 
barely let the tip of the brush contact the coil, do not apply any 
pressure! If you apply pressure, there will be "steps" where the turns overlap.

6. Repeat this end-to-end spreading until the coil is completely and 
uniformly coated. If there isn't enough resin to fully "wet-out" the 
windings, add another very thin "stripe" of resin from end-to-end and again 
level the coating with the brush.

7. While leaving the winder running, very carefully inspect the coating for 
dust particles or bubbles. The winder can be stopped for a few seconds and 
bubbles can be popped with a sewing needle, but I find it easier to gently 
"sweep" the coating with a heat gun or hair drier. This both thins the 
resin and expands the air bubble so they pop automatically. Any lint or 
dust will show up as raised "pimples"; these can be carefully lifted out 
with a needle.

8. When you are satisfied with the quality, slowly leave the room and 
gently close the door, trying not to stir up any dust. Leave the winder 
slowly running for 8 hours. Resist the temptation to open the door and 
"check up" on the coil, and especially to touch the coating to see if its 
fully cured. You WILL leave a fingerprint as a testament to your impatience.

9. Finally, after the 8 hours are up, turn off the winder and remove the 
secondary. No matter how careful you were, there will likely be one or more 
dust pimples. Very carefully touch these spots up using 600 grit wet-or-dry 
abrasive paper moistened with soapy water.

If all went well, your secondary will have a glass-smooth surface that is 
far more scratch and impact resistant than any urethane coating ever could 
be, and will have significantly greater insulation against flashover or 
surface breakdown.

(If you use the Enviro-Tex epoxy resin, their literature will mention 
exhaling on the coating to help pop bubbles, and that it's the carbon 
dioxide in your breath that causes the bubbles to pop. I think this is pure 
speculation, as I have tried a stream of pure CO2 as well as a stream of 
pure nitrogen, and neither worked nearly as well for popping bubbles as a 
stream of warm air from a heat gun. Do not allow the heat gun to dwell at 
any one location for more than a few seconds, as you do not want to start 
to polymerize spots of resin from external heating, you want the entire 
coating to slowly and naturally cure to get the glass-smooth finish.)

With a little experience and a process similar to that described, you can 
get very high quality results with just a little more time than is required 
to apply a single coat of polyurethane. If you are going to apply several 
coats of urethane, the epoxy ends up taking less time and yields much 
better results.

Feel free to contact me off-line if anyone has any questions or comments 
about this process.

Scot Hanson