> 1) Can you tap screw holes into fiberglass? I'd like to make a
> plywood and fiberglass cap for the mirror cell, and have it held
> on by 3 screws into threaded holes, so that the mirror cell is
> easily removeable.
The result of what you are trying will be a tube with a very thin wall. It may be too thin to tap, but you can always use a bolt and nut. You have to be carefull not to tighten too much, though or the glass/resin matrix will crack.
> 2) Is there something magic about fiberglass cloth, or would any
> reasonable stiffening agent be OK? Or, do I even need the cloth?
Not magic, chemistry. Glass cloth is coated with chemistry to help it adhere to the resin. BTW, moisture, especially while cureing, will diminish the ability of the resin to bond to the glass. Keep your glass very clean and dry.
You DO need the cloth, that's where the stregth of the structure comes from, not the resin.
> 3) I'm thinking about getting some pigment from a marine supply
> place, and coloring the tube, to make it some kind of cool-looking
> auto or boat color. But, I'm also concerned about making the tube
> block light. So, maybe a layer of black construction paper
> sandwiched inside would work? Or, maybe a layer of aluminum foil?
> If I was careful with that, it could give a good result.
The paper is not a good idea. It would be the weakest link in your matrix. Same for aluminum foil. I'd suggest puting the construction paper INSIDE the finished tube. This would also help reduce internal reflections. Painting the outside will work fine, but sand it down to removed the very thin outer portion which didn't cure properly because it was in contact with moisture in the air.
> 4) How thick does the resulting tube have to be? I'm currently thinking
> 2 layers of cloth/resin, then a light shield layer, then as much
> resin as I can put on it.
Whoa! The stringth of the structure is in the glass, NOT the resin. The resin is only there to stransfer the load to the glass. The glass is very strong along the fibers and can absorbe huge loads. The resin is brittle. When you construct your layups, you should endevor to use as little resin as possible while keeping the glass throroughly wet. Using too much resin is one of the most common errors beginners make when working with fiberglass. Also, you might want to try using a release agent painted on the outside of the sonotube instead of wax paper.
As far as 'how thick'? You need to do a little engineering work here. The book I mention below discribes many of the desigh parameters you need to be concerned with to make your tube strongh enough for it's inteded use and as light as possible.
> 5) If I have leftover resin, can I use it as the ultimate weather-
> proofing varnish for wood?
Sure, but it doesn't put up with UV very well. Also, you might want to practice doing this before starting on your project. There are several books that discribe working with fiberglass in great detail. One, 'Composit Basics', is very good. I don't remember the name of the author, but if you private e-mail me for it, I'll look it up. BTW, I'm far from an expert on this subject, but I've been building a compsite airplane for about two years now, so I've made enough mistakes along the way to offer a little advice. (;->
Good luck with your prject!
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