I have never had much luck with the Birchwood casey stuff. I have been using Brownell's OXPHO- BLUE and it works well for me. I clean the area with 0000 steel wool then alcohol and warm the metal with a hair dryer before applying. I use the cream formula its easier to control.
if you have not properly cleaned and degreased the area to be blued,it wont take.You cannot simply wipe it on and expect it to work like a hot blue.Also with cold bluing it usually takes several coats.
Oxpho blue works very well. Be prepared to apply several coats to get it to match the original blue.
Use STRONG alcohol and clean cotton to clean and prepare the surface first. Apply the Oxpho blue with clean cotton as well. When you get it to the blue you want, hit it WD-40 to stop the bluing process. Wipe it clean and dry after that.
It works great. It doesn't work on alloys - but if its a Marlin you're working on it will blue well.
I used Birchwood Casey on a couple of levers and they came out just fine; I got it to match the remainder of the lever. You can't argue with success - or failure - if Birchwood Casey didn't work try Oxpho, none of it is terribly expensive. I imagine that you've read up on using cold blue, but surface prep is very important: steel wool has a little oil it it (to stop rust) and that will mess up your blue job, I used denatured alcohol to clean the metal after the steel wool.
Make sure the metal is prepped properly, surface prep is the absolute most important step in bluing. Apply only to the cleaned area, the bluing solution can eat away at existing blue. Don't rub the area until the new blue is cured. It probably won't ever match perfectly and won't get any darker after about 4 coats. Make sure your steel wool is degreased too.
Hope that helps although I do agree that the BC stuff isn't that great for Marlins, I've used it on Rugers with decent results. Never tried the Oxpho
If you have the time and resources, look into slow rust bluing and do the whole thing.
I've used both with good results but Birchwood Casey is available locally off the shelf, while Oxpho Blue is proprietary to Brownells and you have to pay shipping plus an extra hazmat fee. But, in any case, prep and warm the metal with a hair dryer before applying for best results.
That oxpho stuff is available at Cabeler's if you've got one handy. If not, then the Birch Casey works the same. I had to re-blue my entire rifle after doing something that earned me the moron-first-class with oak leaf cluster award. I tried the regular Birch Casey, their stronger solution, and oxpho and it's all equally worthless, unless you get rid of the oil on the metal first.
Getting rid of the oil on the metal, now that takes some doing. Burning it off with an acetylene torch doesn't work because it leaves a brown burned hydrocarbon film, alcohol swabs are nearly useless, even acetone doesn't work that great. the Birch Casey de-greaser is hit-or-miss (like the engine). But Carburetor cleaner on the other hand... Just make sure there isn't anything plastic or wood around. And shut off that torch and nicht rauchen!