potato stuck on 1 open end, fill with hot caustic soda, leave a while and check regularly...it'll be clean as a whistle when its flushed
Did that back in 90's/worked well - left all day,wear gloves!...i did'nt can leave a nasty on your skin.
Not a word you forget,when you get it on your hands/wrist, and don't wash your hands for several hours!. .