Acrylic paint is water-based and thus water-soluble when wet, so water can be used to thin it. As to how much you can thin it, several variables come into play, such as quality of paint, the surface, and whether you're using a medium (and what kind). Some sources advise not to mix acrylic paint with more than 50 percent water. Any more than this may cause the polymer in the acrylic paint to break down and lose its adhesive qualities, resulting in peeling or flaking at some stage or the lifting of the paint when you paint subsequent layers.
To be safe, many manufacturers suggest that you use no more than 30 percent water to thin acrylics when painting on a nonabsorbent surface, such as a primed canvas. When painting on an absorbent surface, you can use any amount of water because the fibers of the unprimed canvas, paper, or wood will hold the pigment to the support as well as absorb the excess water. If you use less than 30 percent water, you eliminate any concerns about having a negative effect on the binding properties of the paint.
Experiment With Acrylics
It is good to experiment and see for yourself what happens to acrylic paint with various amounts of water added to it. Make a color chart and label the wash swatches with the various ratios of water or types of medium used. You'll note that after being watered down past a certain point, the paint starts beading and breaking up into little specks of pigment as it dries. This is showing that the water has caused the acrylic polymer to lose its binding properties, resulting in the dispersion of the pigment. With good-quality materials, you can use a lot of water with your paint to achieve different effects. Higher-quality professional-grade acrylic paints can actually hold more water than lower-quality student-grade paints because the professional-grade paint starts out with a higher pigment-to-binder ratio.
If you do want to thin your paint dramatically with water, it is possible to use more than 50 percent, according to Nancy Reyner, author of "Acrylic Revolution." On her painting blog, Reyner says that she sometimes uses a ratio of 80 percent water to 20 percent paint in what is called "overdiluted" paint. How this paint reacts depends on the surface on which it is being painted. She says that it is best to use high-quality paints on a surface that, if primed, is done so with professional acrylic gesso, and to use filtered water to get rid of impurities.
Mixing acrylic paint with a higher amount of water makes it act like a watercolor paint and gives it more of a matte finish. If you're new to glazing, take a small container and put in some paint and 50 percent water (judge it by volume), then mix the two together thoroughly to get a feel for just how much water this is. Unlike watercolor, because acrylic is not water-soluble when it dries, you can paint layers of glaze without disturbing the underlying layers.
Painting With Mediums
To change the viscosity of the paint dramatically while still retaining its chemical integrity, thin the paint with one of the many different mediums available to the acrylic painter.
You can use many different mediums (glazing, texture paste, etc.) with acrylic paints to give different effects, such as thinning, thickening, adding texture, glazing, or slowing the drying time. You can mix in as much acrylic medium as you like because acrylic mediums have the same resin in them that makes the paint stick. Golden, for example, describes its mediums as "colorless paint."
Some acrylic mediums, such as retarding medium and flow improver, are actually additives, though, and do not have the same acrylic binders that the paints and other mediums do, so follow the directions on the container when mixing them with your paints. Golden Acrylic Retarder's instructions warn that if you add too much of this to your paint, it will not dry.