[Packofcrows]

It's buildup from internals of radiator, tubes..etc. Its also duo to water if you add 1/2 and 1/2. Not all water is filtered the same. Never use water straight from your hose. Upon heating and evaporating, many of the times it leaves a white-ish residue like salt. Also, some coolants have little floating objects in them we rarely notice to stop micro leaks.

Haven't done it on my car, but on my truck I flushed coolant, pressure washed what I could, added coolant again, ran truck for a 45mile round trip, flushed it out again and finally added coolant once more. I still notice stuff like that though. It is a waste of coolant following my actions... There's also these mini filters you can buy. Maybe another member can shed more light on them.