Atmospheric Layering in Acrylics

So I'm back to Acrylics for the next few days- I wanted to start working on a larger piece, but I didn't have the colors for what I wanted in oils, and rather than go spend tons of money on paints, I figured I'd try acrylics. I'm building layers into the atmosphere on this piece, so I thought I'd take a second and talk about how to achieve this.

After first session- maybe 3-5 layers in the background

Here is the base layer of this (large, 38x20") painting. The background is scumbled mixing of 4 different colors- Yellow Azo, Burnt Umber, Ultramarine Blue and Paynes Gray. Doing this in random layers is great, but often builds up too much paint. I fix this by constantly spraying the canvas (fairly liberally) with a small misting bottle for cosmetics. This will cause the background to run a bit, but that's ok, because you're going to paint over the edges when the trees go in.

Same Painting- around 35-40 layers in the background

Here is the same painting 5 hours later- As the layers build up (again, misting bottle and paint, over and over), the atmosphere begins to look realistic and believable. The more layers you put in, the more you can get lost in the painting. with painting like this, it's important to not get frustrated with how things look as you progress, but rather to just keep adding layers, trusting in the process. 

It's worth noting- If you us WHITE in this process, use ZINC WHITE and not TITANIUM WHITE because of transparency: Titanium White is literally the most opaque color out there, and doesn't lend itself well to this technique.