Cooling system cleaning know-how
Attention! Do not try it with aluminium ones - you'd kill the radiator.
So, you've noticed the coolant in the system is dirty. Let's try to solve it.
1. Rinse the radiator with running (up to 3 atmospheres) water for 30-40 minutes.
2. Cover all the pipes of the radiator (except bottom one) and the cap.
3. Turn the radiator upside-down.
4. Fill the radiator with any fizzy drink (4.5L). You can also use still water with citric acid. OR (the best thing) still water with caustic soda (most of tub cleaner contains it - 30%). 40-50 minutes and it's done. Btw shaking the radiator may also help.
5. Please don't use Karcher for washing the radiator body, you'd damage the cells. After you remove all the shit from inside the radiator just rinse it again.
6. Caustic soda is better not to use if you decide to pour fizzy drink or citric acid water before running the engine (this way you can clean also the cooling system). It's safe to run a car with them for few days after which remove this shit and pour normal modern coolant=))