I mentioned the success of making a small fiberglass tube. I decided to go bigger today--and I seem to have learned what not to do.
1. I needed to enlarge the external diameter of the Sonotube that I am using as a mold. It is very slightly larger than 20.25" O.D., and I need something closer to 20.375" O.D. So I started wrapping masking tape around it. It took ten turns. Then I ran out of masking tape--so I switched to traditional duct tape--that only took three turns. Then I switched to a new style of duct tape that is thinner--it took about ten turns.
2. Okay, I wanted to make sure that the mold wouldn't get stuck in there, and that I would be able to get it out again (since I had to destroy the mold that I used for the small fiberglass tube). So I ran it through the table saw, to allow me to bend one side of the circle. To make sure that it held a circle, I then duct taped the cut.
3. Then I grabbed a piece of 3 mil plastic sheeting, which came folded in half, and I thought, "Great! I'll just put this double fold over, and tape it in place!"
4. Then I tried to apply the 8" wide strips of fiberglass to the resin on the plastic sheeting.
What didn't work:
1. The cut in the mold just made it too flexible.
2. The 3 mil plastic sheeting needs to be a single layer, wrapped ten times around. That lets me skip the laborious application of masking tape.
3. I neglected to tape one end of the plastic sheeting to the Sonotube--and with the double layer irregularities, I ended up with a plastic sheet that kept moving around as I tried to apply the fiberglass cloth.
4. The longest piece of the fiberglass cloth was 51" long. The circumference is 64". Adding the next piece where the previous piece ended worked, but it isn't very elegant, compared to working with a single piece of cloth (hard to stretch to a tight fit).
5. It still hasn't finished hardening--and it is too big to go in the oven. It is hardening over night in the garage, with electric heaters warming up the air in there.
6. I ended up applying only three 51" lengths, making for a bit more than two complete turns of fiberglass on the mold. Last night I was concerned that it was going to be too heavy if I made four complete turns, because I was mistaken in thinking that the weight would increase with the square of the increase in diameter. I was tired, and not analyzing the problem correctly. The weight increases directly with the increase in diameter, and with the increase in length. Four turns would give me a 1-2 pound tube. It is apparent that two turns, even once it is completely hard, isn't going to be all that stiff, and four turns is so light that I can afford to go to six turns or eight if need be.
What I have learned:
1. Don't cut the mold to make it easier to remove. If I use 10 turns of the 3 mil plastic, there should be no problem removing the mold from the tube.
2. Use a single sheet of plastic wrapped repeatedly so that you can get a reasonably tight fit to the mold.
3. Tape the starting end of the plastic securely to the mold--perhaps with packing tape.
4. Once you have the fiberglass cloth cut into strips of the correct length, sew them together so that you have one continuous length that you can wind in one continuous action. There's a gross enough weave in the cloth that it should be possible to hand sew the ends together with just a little overlap--maybe go in and out every third hole. This should be strong enough to tolerate the tugging to get a tight fit.
5. Plan on making it long enough to do at least four complete turns on the mold. That's part of why the small fiberglass tube I made worked out so well.
Fortunately, I still have a second, unused Sonotube that I can use. Since these 12 inch sections cost all of about $6.50, if this one gets destroyed removing it, it isn't that big of a deal to go buy another.
UPDATE: Sewing fiberglass cloth is right up there with nailing Jell-O to the wall. I guess I will just have to apply in 51" lengths, using the resin to hold it in place. Unfortunately, I have run short of the hardener that came with this 5 gallon container of resin. This must be a pretty common problem, because I notice that Home Depot sells the hardener both with the resin, and separately as well.
UPDATE 2: Here's a picture of this first attempt with the split tube mold:
Click to enlarge
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