Every time I use a chemical blackener (usually on brass) it ends up a mess. Instead of the metal itself turning black, it just creates a thin, fragile coating that comes off with the slightest touch, showing bright metal underneath. What the heck is the deal??
Also, it's very difficult to get glue or paint to stick after this treatment, despite wiping off the black gunk.
My advice is the following:
1. The brass must be squeky clean. Some form of absasive cleaning must be used. Grit blaster, steel wool, fine emery cloth, etc.
2. Dillute the blackening solution with distilled water. I have used 1 part of blackening to 5 parts of water
3. Do not dunk the part in the chemicals.
4. Apply the chemicals with a small paintbrush or cotton swab
5. Keep brushing the solution around, keep it moving, add more.
This takes time! At first nothing seems to happen, but slowly the metal starts to blacken. My experience is that this way you can get a finish with absolutely no crud. My theory is that when you dunk the part and/or the solution is too strong, a layer of crud is immidiately formed, and blocks any further blackening.
The best blackening in my experience is Birchwood Casey Brass black. Their Aluminum blackening works on brass as well.
By the way, If your brass is meant to look like steel, it can be polished after blackening with great results.
Last: Wear gloves! You will be handeling the chemical infested part quite a lot, and you will absorb the nastu substances through your skin without gloves. Rinse the parts thorougly with water. Micro Engineering say that you can use white vinagear as a neutralizer for their blackening solution, but I cant really see how you can neutralize acid with acid...
Regards, Haavard H