Have you removed the thermostat and done a back flush?
seems to be about the only way to get all the crud out, and
even then you might still leave some behind.
There isn't any iron in these engines that touches the coolant,
except the impeller on the pump, could it be someone put a huge
bottle (or 2) of that radiator stop leak in it and its just all over the place
in your cooling system?
Wild