Best coolant flush

[vectorit]

I am dealing with coolant that is constantly for lack of a better description at the moment, cruddy...

The engine use to run very clean coolant in the radiator, but ever since it sat for years with only water in the system it developed a lot of sediment.

I have already flushed the system for practically an entire day, and after doing that it was very clean and then added a mix of anti freeze.

Well, the radiator coolant is again full of rusty sediment, and I want to try to flush this out again since I want to put a new radiator in. But I don't want to put a new radiator in if the system is full of crud again.

So, my question is this. Other than doing what I did before, by idling the engine for hours with a hose off the radiator drain petcock and a steady flow of fresh water. Is there an additive that one can put into the cooling system to clean out the rusty sediment from the engine, and not create a new problem by doing so?