If I understand this correctly, the poles you are welding are not round at the base, but are hexagonal or some other polygon. When you put on a round backing strip (on the inside?) you have gaps at the points of the polygon, so you have to form the backing strip to match the polygonal shape of the base to wliminate gaps. Is this correct? Is the mating surface on the base also polygonal or are they round? Do you weld on the outside or inside the base?
What is a C weld? What process and filler are you using?
Regards,