flush it reverse to what is normal flow, fill it with hot water and about 1 tablespoon of tartaric acid (mix it in bucket then fill), it will take about 1L to fill it.
Let it sit for about 15 minutes and flush it thoroughly with running clean water for about 10 minutes add some soap solution to neutralize the acid if necessary, fit it within 10 minutes after the last flush with water. Fill with coolant, burp air and you are good to go.
You will see that the acid mixture will be dirty when it drains out. Even if the radiator looks clean from the pipe fittings.