I've had good luck with it. I've also gotten the copper finish when I either rubbed too hard, went over an area already wet or used too much. I generally warm up the piece with a hot air gun before applying. If you're using it for rust bluing try hitting with a hot air gun right out of the boiling tank, dries the water but keeps he barrels warm. Don't know it this really makes a difference but it seems to help me.
Frank