Spirits of salts is hydrochloric acid, commonly sold as brick cleaner from builders merchants. Probably what you'd buy from Wicks is pretty weak stuff, I ordered some min. 36% acid off eBay and have been using it for a couple of years for rust removal. A wash with washing soda afterwards will get rid of any residue acid.
It work amazingly well but take great care!! The fumes are pretty bad so use it outside, wear safety glasses and rubber gloves.
This is the after and before images of a Bantam barrel after about an hour's soak. I have a recently purchased Huntmaster head which will be receiving the same treatment in the near future. I will finish it off with my sandblaster before painting, but it's worth using the acid first as it gets in every nook and cranny much easier than blasting and saves a lot of time!
'53 Bantam, '55 Huntmaster, '61 Bantam, '79 GS 850, 2004 Burgman 650, 2002 Burgman 400