All of the newer rads, namely within the last year or so, use water based flux, so all that is needed is hot, soapy water followed by distilled water rinsing. If you have a rad with algae growth, then hot soapy water is still the way to go. I also have used DOW scrubbing bubbles before and it works great. You just don't want to leave it in for more than a few minutes, and rinse it out completely.