How Much Water and/or Medium Can I Add to Acrylic Paint?

Acrylic paint thinned with water in one sample and Golden Acrylic Glazing Liquid in another sample.
Acrylic paint thinned with water in one sample and acrylic medium in another. Photo by Lisa Marder

Question: How Much Water and/or Medium Can I Add to Acrylic Paint?

"Are there any guidelines regarding what ratio of acrylic paint from the tube to medium I should use?" -- Diane

Answer:

When it comes to thinning acrylics, guidelines vary. It is a water-based medium and water-soluble when wet, so water can be used to thin it, but some sources say not to mix acrylic paint with more than 50% water. Any more than this amount of water 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 lifting of the paint when you paint subsequent layers.

To be safe many acrylic paint manufacturers suggest that you use no more than 30% water in combination with acrylic paints when painting on a non-absorbent surface such as a primed canvas. When painting on an absorbent surface you can use an unlimited 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% water you eliminate any concerns about having a negative effect on the binding properties of the paint. 

It is good to experiment and see for yourself what happens to acrylic paint with various amounts of water added to it. You will see that after a certain amount of water is added, the paint starts beading and breaking up into little specks of pigment as it dries. This is because the water has caused the acrylic polymer to lose its binding properties, resulting in dispersion of the pigment.

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, though, it is possible to use more than 50%, according to Nancy Reyner, author of Acrylic Revolution (Buy from Amazon).

On her painting blog Reyner says that she sometimes uses a ratio of 80% water to 20% 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. With good quality materials you can use a lot of water with your paint to achieve different interesting effects.

Mixing acrylic paint with a higher amount of water makes it more like a watercolor paint and gives it more of a matte finish. If you're new to glazing, I recommend taking a small container and mixing some paint with 50 percent water (judge it by volume), then mixing 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. 

 To change the viscosity of the paint dramatically while still retaining its chemical integrity try one of the many different mediums available to the acrylic painter.

Mediums

There are many different mediums that you can use with acrylic paints that give different effects, such as thinning, thickening, adding texture, glazing, or slowing the drying time.

You can mix in as much acrylic medium (glazing, texture paste, etc.) as you like because acrylic mediums have the same acrylic resin in them that acts as the 'glue' that makes the paint 'stick'. (Golden describe their mediums as 'colorless paint'!)  To create very thin paint without changing its adhesive properties, you might try mixing it with Liquitex Professional Glazing Fluid Medium (Buy from Amazon). Other manufacturers also make glazing medium.

Some acrylic mediums are actually additives, though, and do not have the same acrylic binders that the paints and other mediums do, so you want to be sure to follow the directions on the container when mixing them with your paints. Retarding medium and flow improver are actually additives rather than mediums so do not contain acrylic binders.

Golden Acrylic Retarder (Buy from Amazon) is an additive that warns that if you add too much of this to your paint it will not dry. Golden Acrylic Flow Release (Buy from Amazon) is an additive used to reduce surface tension of the water in the acrylic emulsion, thereby increasing the slickness and flow of the paint. Read about Golden Paints additives here.

Further Reading and Viewing

Worst Mistake Acrylic Painters Make (video)

Acrylic Mediums Part 3: How to Use Acrylic Glazing Liquid and Mediums (video)

How to Make Your Own Fluid Acrylics

Introduction to Acrylic Mediums

 

Updated by Lisa Marder 6/11/2016