Ah Sir, This is something I have been doing a lot with lately. Here is how I do it. Buy common hardware store Muriatic acid(HCl 20%), throw in a small handfull of forge scale(iron oxide) and some water to dilute. it will turn black greenish(forms iron chloride?) Put it in a plastic pan of some sort(big enough for your work) that you can cover with a piece of glass or plastic. make some sort of wooden rack to elevate work above level of liquid. Clean work well, degrease. Place on rack over acid, cover, go home. next day it will be covered with spotty rust. using an old paintbrush(natural bristle) scrub acid mix onto surface wiping off to leave a thin film. place back into pan on rack, cover and go home. the next time you look at it it will be covered with an EVEN brown layer of superficial rust. Scrub off with FINE steel wool and oil. Ta Dah! I have gotten very good results doing this, a nice even black-ish patina. If you skip the first step of letting the work start from just acid fumes, it is very hard to get it even. have no idea why. discovered this by empirical trial and error. mostly error. You CAN just slop acid on and let it sit in open air, but the results are very unpredictable.
