If you would use any of the better cold blues on the market, then David is spot on about getting the base metal completely degreased. Auto brake cleaner is about the best (outdoors!) Personally my cold bluing days are over. William R. Brockway, author of "Recreating the Double Barrel Muzzleloading Shotgun" gives a very detailed text and pictures of his cold rust bluing set up. Simple and inexpensive, only a bit labor intensive. This paperback book contains a wealth of info that can be applied to many early double barrels, and was available online at least fairly recently.