similar here-  After  using  both  cascade then a  commercial  radiator flush ..   I took the  thermostats completely out (they were shot anyway)    and  kept the  top hose  off,  using  a  garden hose   refilling the  radiator.      



 

This was for the  final rinsing part.      10 minutes of  fast idle,  the  water was  perfectly clear.   Gets a little messy with  water  slinging off the fan. 
But that was on a nice hot summer day.   Definitely  an outside job.