I used radiator stop leak in my car becasue of a bad heater core. I have now replaced the heater core, but it has since started overheating, and the coolant hasn't been changed yet. I plan to replace the harness end that connects to the sensor(seems to be corroded), and then change the coolant which has turned to sludge and needs a desporate flush. The main problem I see is the fans never turn on. also bad coolant is inefficient for cooling a car anyhow.
It needs a acid flush, such as CLR or vinegar due to sludging.