Best way to clean calcium build up out of a radiator??

I've been meaning to try the CLR stuff, but haven't yet. What I have used is a product called Tub and Sink Jelly. It's in maybe an 8 ounce bottle, and like the label says, is a jelly. I mix 2 bottles with a gallon of hot water, and with the inlet and outlet of the radiator blocked, (I stretch an inner tube over the necks, and put on a band clamp to seal), I pour it in, replace the cap, and tip the radiator back and forth, upside down, etc to circulate it. Within a minute or so you will hear a clinking noise, and that is the calcium chunks coming loose. I do it about 5 minutes, and rinse it out well. The few I've done look like a new penny inside. Distilled water only afterwards.