Re: fiberglass plastic toroid

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Original poster: FIFTYGUY@xxxxxxx 

Folks-

    I made three toroids in the past year. My previous two were both 
plain ol' aluminum duct with Al foil. Cheap, easy, practical, but not 
pretty. Also electrostatically leaky!
    To get a smoother finish and some strength, without resorting to 
paying for spun toroids, I decided to go the "Bondo" route.

    1. All toroids use aluminum duct for structure, with 3/4" 
urethane foam board center (foam board has a heavy aluminum foil 
covering). All toroids finished with heavy-duty aluminum foil outside 
layer, spray adhesive used to attach, and burnished with the back 
side of a spoon.

    2. I tried the "Water Putty" stuff. Very friendly, cheap, and 
easy, except it doesn't stick to bare metal and flakes off in chunks. 
Also affected by moisture/humidity.

    3. I tried thick aluminum roof paint to fill in the duct 
grooves. Nasty stuff, goes on thick, dries much later too thin. Not 
even sure if it conducts enough to work bare. *Maybe* OK as a final coat?

    4. Tried Bondo body filler. Adds a lot of weight, but results in 
an indestructible toroid!

    5. "Clear-coated" the smooth body filler with pure resin. Didn't 
dry very smooth at all. *Very* difficult to sand resin, as it gets 
gummy and clogs sandpaper/tools. Lesson: Just as good, if not 
simpler, cheaper, easier, and all-around better to use plain Bondo 
with no clear-coat over.

    6. In an attempt to save weight, I used expanding foam to fill 
the duct ridging. Very cheap, I used the old "Mountains in Minutes" 
brand two-can manually-mixed type, not the gap filler aerosol stuff. 
Either way, put it on thin but ugly, sands or Surforms down very 
quickly. Go back and hit the spots you missed the first time. Fills 
wonderfully.

    7. Have to make sure to put enough Bondo on over the urethane 
foam, otherwise you create a thin shell of Bondo that's easily cracked.

    8. I just weighed my 8x36 toroid, 8" duct, urethane foam ridge 
fill, Bondo shell, Aluminum foil covered: 6 lbs.

-Phil LaBudde