Ok here is my way.
Put 2 Big Spoons of SURF Excel in ur cars radiator.
Drive for 1 whole day flush the water put new water. Drive for 1 day.
Take out Radiator.
buy any bathroom cleaning Taizab Easily available. now if its too strone mix 40 Taizab and 60 Water in a bottle and make sure ur wearing some gloves or avoid contact.
Now put that water on Radiator Fins from out side, Wait for 5 Mins.
Wash with high pressure but try not to damage the fins by too high pressure.
BTW this is a desi totka so risk apna apna 