Folks... I didn't want to tell this story. It's complicated, inconclusive, and partially second-hand. But since at least two people in this thread are not completely aghast at the prospect of dumping a highly acidic household cleaner into their radiator, I feel duty-bound to share what I know of the use of CLR in a cooling system.
I work at a Detroit Diesel dealership. The other day a truck driver comes in... his truck is losing coolant at an alarming rate: a couple gallons a day, no external leakage. The engine is question is a Series 60 Detroit- 500hp, inline six, direct injection, overhead cam... a fairly modern design. I think the truck was about a '99 model, and had an aluminum radiator w/ plastic tanks. He relayed this long, sad story:
Several days previous, the truck had been running kinda hot, and he suspected that the cooling system needed a good flush. On the advice of a buddy, he put CLR in the radiator- quantity unknown.
After a day, the coolant was a dark, soupy, rust-colored substance. Thinking that the CLR had worked (and I reckon it had, in a sense), Super-trucker drains the rust-soup, flushes the system with water, and refills with new antifreeze.
A couple days later, the coolant is again a rusty/soupy mess. He flushes again and refills with coolant.
Again, the coolant turns into a rusty soupy mess.
Understandably alarmed, Super-trucker called up the same buddy who initially advised the use of CLR. Buddy said to use Red Devil lye to neutralize the CLR. Super-trucker did as instructed.
I don't know the immediate results of the Red Devil Lye... but suffice to say that a few days hence, Super-trucker is in our shop telling all this to me.
The truck had well over a million miles, and under Detroit Diesel's overhaul program, didn't qualify for much of a warranty if overhauled. His only option to get a reasonably warranty was to go with a reman engine, which we happily installed to the tune of ~$15k all said and done.
Unfortunately, I didn't get a chance to disassemble and inspect this engine- it would've been informative and good for a laugh. But in a Series 60 Detroit, there are only a few possible causes of this sort of coolant consumption: Cracked head, leaking injector tube, blown head gasket, or cavitated liner... that's about it. Granted, the engine had over a million miles and had been overhauled at least once before, but I'm pretty confident that CLR in the cooling system contributed to a significant mechanical failure... to the tune of $15k.
So my advice: Don't do it. You'll be sorry. Just like Super-trucker.