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Re: fiberglass plastic toroid



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