I'm using cold pressed watercolor paper. It's a little tricky to spray on but I have the option for using brushes and sponges with water color paints in addition to air brushing. At some point I'll try airbrushing on a different substrate.That's a rough painting surface, is it something you had laying around?
I tried what jackEb , the dragon hunter hinted at on my other project... brushing the background before the subject... it seemed to work a little better on this project so far. I'm getting better control on the airbrush as I am learning where it starts to spray etc... Also getting better with thinning the paints. You are right with the layering... it makes a huge difference.Looking good so far!
You appear to be building the color and saturation slowly, which is good. One of the hardest things to overcome in the beginning, at least for me, is understanding that your first go at it doesn't give you the finished product. It's going to look ugly at first, but the layering is just building up to what you want to see in the end. You are constantly refining the image until you are happy with how it looks.
Keep at it!