Thin
Instead of using water to thin out your acrylic paint, try using airbrush medium.
Particularly in under-painting, and particularly with cheaper brands or student lines of acrylics, too much water can break down the bindability of the paint to the substrate, and eventually lead to the painting literally cracking or peeling off the substrate.
Using a high flow medium, like airbrush medium, gives you the liquid consistency you're looking for without breaking the bindability of the paint.