More Fun With Fiberglass
I made another try, wrapping the fiberglass glass radially around the cardboard tube, instead of applying it to the exterior face. I can't say that it made any better of a surface, but it means that the interior of the tube is now too small for the 20" OD mirror cell that I was planning to use.
The good news is that the fiberglass does definitely stiffen these tubes substantially with only a small increase in weight. The bare tube weighed 2.5 pounds; after applying the fiberglass cloth radially, it is now 3.5 pounds--and is obviously much stiffer. I'm not sure that it is quite stiff enough for a telescope tube, but perhaps one more layer (on the outside) would do the job, and only get the weight up to 4.5 pounds per section.
I do think it might be worthwhile to try and buy the thicker Sonotube from the operation in Bozeman, Montana, even with the shipping charges, just to get the extra stiffness. Perhaps I won't need to use fiberglass cloth with the thicker Sonotube--maybe just applying the resin inside and outside will be enough. Since the thicker Sonotube weights 3.3 pounds/foot for the 20" ID form, the tube will be 2.2 pounds per section. With the resin applied, even in two layers on the outside, this should still be less than 4 pounds section.