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

You ever put a chicken bone in a bowl of vinegar? After a day it turns to "rubber". I've never really had CLR work on anything I tried it on. Their warning about brass/copper is probably related to discoloration.

If the radiator is out, connect an old bike inner tube between the inlet and outlet to seal and then fill it with straight vinegar. Won't hurt anything but if there's calcium inside, it'll dissolve it. Let it sit for a day or two.

(Hydrogen Peroxide combined with vinegar purportedly dissolves lead).