Phosphoric Acid (85%) is the most useful substance I've found. You have to be VERY careful as it's a strong acid (and a syrupy fluid), but it can be used for all sorts of descaling/de rusting applications with only very small amounts.
Add to water and put in a kettle and leave and limescale will be gone, if you have rust, then just apply a small amount and the rust will dissolve and the iron phosphate left will form a protective layer.
Will remove limescale that other products can't touch.
Again be very careful as it's a strong acid.
