My 2001 Blazer with the cast iron 4.3 had severe rusty coolant issues, from Dexcool and tap water being used by the previous owner. I’m talking about radiator cap, radiator core, and heater core being completely plugged with gunk and sludge to the point of overheating after sitting for a couple minutes. I flushed it one time with prestone radiator cleaner and distilled water about a year ago, and I thought I was gonna be good, but the rust was back after a year of driving. No more sludge, but rust was still present.
I feel the only way to really take care of rust formation in the cooling system is to actually chemically remove all rust from all the individual cooling system components, especially in the iron block.
I used Thermocure Evaporust and that thing actually completely removed all the rust buildup on absolutely everything, inside the radiator, the inside of the block, and everything. Everything looks brand new on the inside now. I drove with a gallon of Evaporust and distilled water in the system for 3 days, and what came out afterwards was shockingly nasty. When draining, it Smelled like nasty heavy rust, but this stuff really cleaned out the whole system 100%. I can now see bare metal inside the radiator, and even inside the block. Looks like brand new clean cast iron.
After all that, I flushed the whole system individually (radiator, block, heater core, all the hoses) with many many gallons of distilled water, and filled the system with 56/44 prestone max coolant.
Hopefully this stops the coolant from becoming rusty brown looking only after a year of driving. I feel left over rust in the system is what causes coolant to become rusty looking, so removing all the rust and putting in good antifreeze will stop rust from forming. Having any rust will eventually cause even more rust, even though coolant should stop corrosion. Gotta remove it all to stop it from forming.
