Everything Keith says,,,,
You have to etch the rusting cycle,,or the rusting and boiled cycle (depending on wether you are after brown/white or black/white).
The etch is what weakens the color layer from the steel (??) laminates of the make up of the twist and allows them to be carded off while the color stays put, for the most part, on the iron portions.
You do loose some off the iron with each etching also, that is why it's such a slow process to build up the final finish as compared to a simple rust blue or rust brown.
I use a less than 5% ferric chloride solution for the etch. Probably more like 3%. Nothing I mix is very precise but I know it's at least that weak. Too strong and it'll wipe off everything or most of it in a simple in-and-out immersion dunk in the stuff.
It takes some time to find a technique that works for you. It might not be the text book written instructions that someone else has provided.
Like so many other finishing processes, the variables are endless and the small little things we do w/o even thinking they may have anything to do with success or failure often really do.
