I could slide a piece of rebar through the middle of the pipe and then pour fiberglass inside. No way in the world that would ever flex. |
Most of the strength and rigidity comes from the outside, so adding rebar and epoxy down the middle would have little if any effect on rigidity. If you need more rigidity go with larger diameter pipe, or else continuously supported pipe. Most people grossly underestimate the amount of flex in rails that are only supported at the ends.