Hydrochloric acid works well, but it has the disadvantage that if you forget about it, the parts will be attacked/disappear; it also attacks your skin and is hazardous waste once you have finished with it.

I prefer molasses, mixed 1 part in 10 with water. It will attack steel, but does it so slowly it doesn't really matter if you remember weeks later, that you have something in the bath. For this reason, like hydrochloric acid, it needs to be in a plastic bath of some description; bucket; drainage pipe; black plastic etc. It also has the advantage of not attacking your skin but I wear gloves anyway because the decaying rust pongs a bit; it is organic and non-polluting as far as I can make out.

If you mix it with hot water and keep the bath warm (as when brewing beer) it will work very quickly, especially if the parts are removed every 3 or 4 days and scrubbed with a wire brush.

Nice work Stuart!

Cheers Charlie