You can use this stuff to seal slip joints. Just apply a layer to the inside of the female joint with your finger.
As for the joints rotating, I just got mine into place where I wanted it last night, clamped down the joints, then drilled a hole in each slip joint and inserted a small sheetmetal screw with a dab of the Ultra Copper on it.
Neither of these fixed the fact that the muffler hanger positions are way off but it's sealed up and not moving around anymore.