I have over the years flushed out a couple of rads both main and heater matrix and have dull memories of flushing out an engine while I had the rad out for flushing.... The thermostat would need removal first. and after the initial surge of crud reversing the direction of flow seemed to dislodge a tad more crud, which encouraged me to carry out the reversing process a few times more till the outflow was clean.
It also seems a bit like oil choices, one may require to be selective when choosing antifreeze types given all that seems to be available now..?
Good luck.
