I use MM enamels for my base coats, usually apply future, put on decals, and seal the whole thing with Dullcoat before putting on my wash.
Normally mix my wash in a standard MM size glass bottle. I use a mix of about 6 - 12 drops of acylic black or burnt umber paint (depending on the darkness I'm looking for), mixed with enough water and liquid dish soap (Dove is my choosen brand since it's fairly clear) in a 50/50 ratio to fill the bottle. Yes, I said 50/50. The wash is fairly thick. I apply it where I want and let it dry nearly completely - usually 6 to 12 hours. Even after this amount of time, I can then take a Q-Tip or a brush moistened with plain old water and remove any wash that has gone where I don't want it or is to dark. If I really screw it up, all I have to do is soak the entire model in some water and 90% of the wash will just come off and I can start over.
Once the wash is just the way I want it, I'll let the model sit an additional 12 - 24 hours to dry completely, then seal the wash with another layer of Dullcoat.