When it comes to this, few know more than our technicians at HB Auto & AC. Chris worked on cooling systems almost exclusively for over 20 years one gets a pretty good feel for a lost art. We've detected more over heating problems, coolant leaks, head gasket failures and the many other "radiator" and coolant related problems than almost anyone, PERIOD.
This has given us a perspective on vehicles that carry over especially in reference for maintenance. Nothing will deteriorate and become like MUD more quickly than a failing radiator and or cooling system. There is a progressive corrosion, electrolysis or acidity that can classically happens in a radiator or cooling system. They always need metering or proper maintenance. In a similar way, we've discovered that ALL AUTOMOTIVE FLUIDS can have their own electrolysis, acidity or chemical reaction. That's one of the reasons motor oil is changed on a regular basis.
My favorite fix is welding an aluminum radiator (or AC condenser or AC component) in the vehicle (saves labor of removal and reinstatement), to save cost of part and labor of replacement. We still get jobs in our laps to repair a bump spot that happens even when the best of automotive repair shops mistakenly poke a hole in a BRAND NEW RADIATOR or AC CONDENSER! There is a trick to do it and we know how.