A few other points:
- The idea of using a sonotube for a mold is a good one; just be sure you can get the mold out when you're done!
- Resin does not stick to mylar; consider using a sheet of it to cover the mold.
- Howard's book on telescope making has a chapter on making tubes, but I don't think much of his methods. In particular, only a madman would make a mold the way he does.
- Boat hulls are made with an EXTERNAL mold, which is first sprayed with a special (and expensive) type of resin called "Gel Coat", followed by overlaying the glass cloth. I believe epoxy paint works about as well.
- UV light quickly ruins unprotected resin, so it wouldn't make a good finish for anything that will be outside.
- There are descriptions of FG tube making in a) Advanced Telescope Making Techniques Vol 2, b) S&T Nov '91.
Larry Phillips
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Fiberglass 101
Fiberglass structures consist of fine filaments of glass embedded in hardened resin. The remarkable strength of the material comes mostly from the glass; the resin simply bonds the filaments together. In most fiberglass construction, the glass, often in the form of woven cloth, is attached to some type of mold. After adding a catalyst to the liquid resin, it is then used to saturate the glass. When the resin hardens (which doesn't take long), the mold is no longer needed.
The glass comes in two basic forms:
Matt material consists of randomly oriented short fibers pressed into a flat sheet. The fibers are coated with an agent that loosely bonds them together. This bonding agent dissolves in the resin, making it easy to shape the matt material into compound curves. Also, by purposely fraying the edges of the matt, two edges can be joined to form a nearly invisible seam (useful in a telescope tube). Matt is rated in ounces per square foot, with 1 1/2 ounce being typical.
Cloth material forms a smoother surface, and is slightly stronger than matt. It is also less frustrating to work with, since it holds together better when the resin is added. Cloth is rated in ounces per square yard, with 15 ounce being typical.
Both matt and cloth are sold like fabric, by the running yard. My supplier had 40 inch wide rolls.
Resin also comes in two varieties, polyester and epoxy. Polyester is the type commonly used; it works fine and is far cheaper than epoxy. Polyester resin is a syrup-like liquid with a very strong (but presumably safe) odor. Sufficient catalyst, (MEK Peroxide), must also be purchased, in the amount of one to two ounces per gallon of resin.
When catalyst is mixed with the resin, an irreversible reaction begins in which the resin changes to a hard plastic. Under normal conditions, it becomes too thick to work with after about 20 minutes, and fully hardens within a few hours. The time is strongly influenced by ambient temperature however; the cure time at 85 deg. F is about half the cure time at 70 deg.
When ordinary resin is used, the oxygen in the air inhibits the outer surface from curing completely, leaving it slightly tacky. This will help a subsequent layer to bond to it, but it also makes it very difficult to sand because to tends to clog sandpaper. I recommend using "waxed" resin, which has a small amount of paraffin dissolved in it. The paraffin migrates to the outer surface, sealing it from the air during curing. Just be sure to remove the wax prior to painting by wiping down the surface with acetone.
Inevitably, some resin will get on the hands or on clothing. Before it cures, acetone dissolves it fairly well, but cured resin has no known solvent. It is probably best to wear rubber gloves, although coating the hands with Vaseline before starting works fairly well.
For finishing the tube I used, and recommend, a two part epoxy paint. It can be applied directly over fiberglass (no primer required), and it dries to a hard, porcelain-like finish. In fact, one of its intended uses is to resurface old bathtubs. When the paint has dried, the cardboard tube and disks can then be pushed out.
Materials
My tube had a surface area of 21 square feet, and required about 1 1/2 gallons of resin to form 2 layers. Resin and glass are available from auto parts stores, but they sell resin only in quarts, and the cost is exorbitant. Resin can be bought from a fiberglass supplier for about $21 per gallon, including catalyst. Sherwin-Williams and Wal-Mart are two sources of epoxy paint, but they stock it only in white.