I also question the idea of grounding the outside of a plastic pipe. The plastic piping systems I've seen advertised suggest running a wire down the inside and drilling a hole through near a connection so you can attach the inside to a bolt and another wire to the bolt on the outside. My preference was to use galvanized ducting instead. It's cheap and easier to ground. I still used the bolt idea but no wire needed on the inside. I also still used plastic ducting at the corners instead of galvanized bends. The plastic ones are much smoother. Use the aluminum foil duct tape at all joints and it works just as well.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.