I never use an airbrush....
I also don't thin my paints. Some of my models are displayed in the LHS, and people think I airbrush.
You can dip in the jar, just make sure you don't have a ton of paint on the brush. Going with alittle paint at a time (almost drybrushing) for the first coat or so. I usually end up with 3 coats, and the model is completely covered...no brush strokes.
As for white, it takes me an average of 10 coats of paint to get it to look right. Just make sure you only put alittle bit of paint on at a time, otherwise you'll get that *too thick* affect you were speaking about earlier.
I bought afew cheapy no-name company models to work on at first, to get used to painting. That way you don't screw up on an expensive build.
Good luck, and if you have any questions, feel free to email me.
m_24expert4life@sbcglobal.net