Blitz is preferred for sure, but even Mayhem has mentioned alternatives to it before. I do agree though, just trusting room temperature distilled water to do it for you is a bit to naive. Boiling distilled water is better and will at the very least remove the flux and most of the debris from the radiator. To remove any paint residue and corrosion you will have to use stronger chemicals.
It's not going to be as good as Blitz, but you could try it the way Mayhem shows it @17min in this video:
A.) should've stuck with what was working
B.) who knows what weird stuff is going on now in the loop if you had auto coolant going before, & didn't clean properly.
you need to do a genuine flush. not just fill & drain.
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R.I.P. Zawarudo, may you OC angels' wings in heaven.
If something appears too good to be true, it probably is.
do not use any compounds or solvents on the nickel itself.
radiators are traps. they're the main thing you wanna acid bath, then basic bath.
Should be fine just running massive amounts of water through it to. I have for example just straight up hooked my radiator to the sink and let it run for an hour, flusehed out the acid/base quite well. Though you should obviously do a PH check afterwards and then rinse out with a less agressive cleaner (Like Sysprep, Blitz part2 or any otehr prep fluid really).