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Elmers GlueLess work - just paint it on and let dry then peel and look at the neat things sticking out of it/
I think I would try that before the sand. I have filled metal tubes with sand and did not like the results. The give of sawdust may work better even were it packed some in the tubes I bent.
For a thing to burn, Chlorine is a more reactive oxidizer than oxygen.I do not know what might be produced from PVC that may burn in absence of air. I don't even know any list of things that can be oxidizer.I think I read that hydrogen and chlorine will just explode - or burn - to produce HCL in same manner that oxygen and hydrogen can be burned to produce H2O.Just thought I'd throw that in .....