Cap off one end, fill like 3/4 with gunk hydroseal 2. cap off other end. Let sit over night. next morning shake the pipes around for a while. Then drain the pipe. Use kerosene to wash the instead out. If possible, use a pressure washer and shot it in there all around. Then wash again with kerosene to stop rust from happening.
This should get some out, but to get all of it out, I am not sure.