In my experience you really should fill them with tap water and leave them for 24-48 hours. After which, empty out a little water and shake the rad for 3-4mins. You'll see loads of flux come out.
By the way, even after all the flushing I've done, I'm still letting the radiators stand with water in them and then emptying them out after a shake - I'm still getting flux!
I'm sure my radiators have an infinite supply of flux in them!





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