Here's a quick tip on airbrush penetration and concentration...
First 1-2 coats use the thinned to 50%, this will help the first coat penetrate better (more water or alcohol) and open up the pores.
Next couple of coats use full strength. On the full concentration coat following the thinned coat, don't wait for the thinned coat to dry completely before the next coat.
Airbrush tip: lowering the air pressure below 'normal' recommended values (30psi for bottom feed, 15-18 psi for gravity feed) to about 1/2 and then using the brush at 3/4 open will create a similar effect as a HVLP gun...more dye, less air so it atomizes less & penetrates more.
For a stipple type fade, lower the air pressure to 1/3 of normal and hold it back a bit farther, you'll get a stipple or granular pattern like the black spots in granite.
Here's an example of a quickie two-tone fade I did last night using the techniques above with Fiebing Black over a custom mixed Cranberry/Burgundy with Eco-Flow. (Yes, that is spirit based over water based). The lighting contrast is a bit off (they are actually much darker) but it illustrates the idea well enough.
~Chris