Fun With Fiberglass
Okay, I'm trying again, this time to make a fully fiberglass tube--not fiberglassing a piece of Sonotube. But since the last experience was so unpleasant, I am starting small (using this set of instructions).
I started out with a paper towel roll cardboard tube, wrapped it in Saran wrap, which is taped to itself. (The theory is that you should be able to pull the cardboard tube out of the plastic on which the fiberglass has set.) The cardboard tube will be used strictly as a mandrel around which to wrap the fiberglass--and only the middle of the tube gets used for this, providing a non-sticky, non-messy section to grab and turn.
Then I coated the outside of the tube with resin, and did a four times wrap of fiberglass cloth, applying resin liberally as I turned it. While the cardboard tube wasn't perfectly round (my wife fished it out of the trash compactor), the surface of the fiberglass came out surprisingly smooth and glassy--and will probably look even better after it hardens and I sand it.
Click to enlarge
Even with the difference in scale factored in, this was much easier than trying to fiberglass a piece of Sonotube--so much easier that I am now prepared to seriously consider using Sonotube as the mandrel around which to form the tubes that I need.
Now it just needs to harden. It's unfortunate that I can't put it in the oven--I understand that most of these resins cure remarkably quickly at 185 degrees--but the wife can't stand the smell, and putting this sample in the oven just isn't going to make her happy! A rotisserie--that would be perfect--rotating and heating at the same time.
UPDATE: After a few hours sitting next to a forced air electric heater in the garage (and after my wife figured out a way to set up reflectors to force more of the heat back onto the tube), it was sufficiently non-pungent that my wife relented, and let me bring it inside, and cure it in the oven at 170 degrees for ten minutes. And wow, it went from pretty hard where the heated air had been hitting it to absolutely solid over the entire length.
Removing the cardboard tube wasn't quite as neat as the pictures linked to above suggest. I had to crush the cardboard tube, but once I did, it popped right out. The Saran wrap mostly came out, but some ended up on the inside. I couldn't very easily get inside to sand it out. I was able to smooth it on the outside with a belt sander.
Click to enlarge
As I mentioned, the cardboard tube wasn't all that round, having been through the trash compactor, so don't let the shape make you think this was a failure. It wasn't! The tube averages about about .090" thick, is a bit over six inches long, 1.75" ID, and weighs just slightly more than one ounce. It is incredibly stiff, and strong enough that I don't think it would break unless I intentionally crushed it between both hands.
I'm not sure how this is going to scale up on weight. The weight will increase directly as the length increases. I think (in my exhausted, ready to go to bed state) that the weight will increase with the square of the increase in diameter, which would mean the 8" long section I need would weigh 11 pounds--which is way too heavy. However, this was four turns, and I've read that three turns of fiberglass cloth is as much as you need to make a telescope tube. It is certainly clear to me that this little tube here is far stiffer and stronger than I needed. Two turns would be about a 5 1/2 pound tube section. I could live with that.