The problem with acid cleaning anything is getting the acid on the surface. Any kind of grease, oil, etc. prevents this from happening. That's why the old time radiator flushes started with a caustic solution, removes all the petroleum products. As a substitute I'd try washing it out with a strong solution of Dawn dishwashing soap to get it squeaky clean and then hit it with some type of acid for the deposits. CLR sounds like a good choice.