Strengthening tubes

Fibreglass it. Painting with epoxy will make it heavier and not a lot else, certainly not stiffer.
may be of interest.

I was thinking of putting a stocking over it and painting with thinned fiberglass resin, but I'm not sure if that would work.

Not sure that would help you much. Fibreglass cloth is pretty cheap and easy to work with, even a single wrap will add noticable stiffness to a thin tube, without adding too much weight. Composite reinforcement is a huge subject on its own, there's plenty of info on using glass, carbon and the like for reinforcing rockets online. Try for starters. Two important things: don't use too much resin and never sand into the cloth.
What size motors are you looking at using? Estes model rocket tubing is very lightweight and surprisingly durable. Salvaged tubes can often be very poor in terms of strength to weight, I have found. Some aren't bad however...
HTH,

Nylon stocking material won't do you any good, as it won't bond to the epoxy -- it'll just be trapped inside the epoxy, so it won't give you any added strength.
If you want some improvement, use CA.
That said... Unless you're really going to abuse it, any of the mailing tubes I've seen are just fine without any reinforcement for the motors appropriate for that size rocket.
If you feel you NEED to reinforce it, use fiberglass.
-Kevin

"Mike" wrote in news: snipped-for-privacy@corp.supernews.com:
stockings are nylon,and epoxy would not bond well to it. If you buy boat-building epoxy like West System,System Three or RAKA,it does not need thinning.Thinning weakens epoxy. System Three has a book;The Epoxy Book,that is VERY informative and worthwhile. They have an online copy of it you can read.(and download?)