I've had good results with Pilkingtons. A 10% ferric chloride solution works well when you boil the barrels to do a black/white finish, but when browning, I think that the rust is easier to remove than after it's been boiled. With that being said, I have been using a solution of about 3-5% for etching a brown barrel, and only etching about every three rusts.

Here are the results. Good luck with your barrels... Craig