CLR can get off any scale but can also eat into the zinc coatings pretty quickly. Also try MetalRescue or EvapoRust to remove all the rust; they can tolerate being left in that stuff a lot longer, but not too long otherwise, as with CLR, can start to blacken. (I prefer the consistency and performance of EvapoRust a bit better but both are good)
evap.jpgmetalrescue.jpg