We used sand filling to control the bends in tubing for building sail planes and restoring antique planes.
It works really well in thin wall tubing.
The procedure is to plug one end with a piece of dowel and cross bolt. pour in fine sand and remove all the air and pack it tight, then plug the end by driving a dowel in hard and cross bolt.
You can bend the smooth bends, no wrinkles, no ovaling.
