Keep in mind that most cold blues will cause permanent blotching of the original blue around the area where it's applied.
I dunno, I've been using it for years, and never seen this problem. At first I only used it for touch up, but I've actually done almost entire guns with it (Old Milsurps, not worth a "real" bluing job). And what I have done has held up pretty well.
The way I do it is such:
1) 0000 steel wool to knock down any proud metal, and slightly blend surrounding bluing. Done VERY lightly.
2) DEGREASE--be ABSOLUTELY sure there no oil or anything else in the area.
3) wipe on the blue, concentrating on the worn area but extending outward. Rub it in then IMMEDIATELY remove any excess.
Repeat the above, until you have the right color, usually 3-4 applications.
When finished buff very lightly w/ 0000 steel wool.
I have an old Mauser that did the entire barrel like this some years ago. I buffed it all down to smooth it out, but there was probably 50% blue remaining. It has been in and out of gun cases, many trips to the range, and sometimes set for a few days, without cleaning, after shooting. Looks good as new. I've even had compliments ("Where did you get that blued?") on it.
