I've never used Plum Brown but know guys that have. Some of their jobs turned out well, and some were blotchy. Same as any other method, I guess it depends upon technique and attention to detail. I have only done slow rust browning using formulas from Angiers book or John Bivens. How old is your bottle? I recall reading that the early version contained mercuric chloride. I think the current version is mostly dilute nitric acid, so either way, I doubt if it would lose effectiveness from just being old. I have read that the fumes from it will clear out a room in a hurry if it is applied at the full 275 degrees F. Supposedly, it works just as well if it is swabbed on barrels that are brought up to temperature in boiling water and then applying the solution as soon as the hot barrels have flash dried.
