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How to Thin Acrylic Paint

In Acrylic Painting by Chris BreierLeave a Comment

Some painting techniques work best with a paint that has a thin consistency, so you may be wondering how to thin acrylic paint. The ideal solution is to use an acrylic paint that already has the consistency you desire, so adding water won’t be necessary. This will ensure the colors will remain bright and the paint will adhere to the canvas.

If you still want to dilute your acrylic paint, it can be thinned with water. Most paint manufacturers recommend a maximum dilution of 25% water. It’s best to use distilled water if you plan on storing unused paint to prevent mold growth (see Does Acrylic Paints Go Bad for more info). Acrylic painting mediums can be used to thin acrylic paint in unlimited amounts.

Why Dilute Acrylic Paint?

Watercolor Effects

The most common reason artists thin acrylic paint with water is to improve how it flows so that it can be applied in thin washes similar to watercolor. The thin washes of color will spread into other wet areas of the canvas so the colors can mingle on the canvas. These effects are nearly impossible to paint by hand.

A detail of an acrylic painting that uses thin acrylic paint like watercolors

Click to enlarge. In this detail of my Painting “Akron Falls State Park” I used thin acrylic paint in a style similar to watercolor. The fluid consistency makes it easier to achieve soft edges and gradients.

Diluting the paint will make it more transparent, allowing the previous layers of the painting to show through. This is a form of glazing. Adding white to a color tends to make it less vibrant, and turns it into a “pastel” color. If your acrylic paintings are turning out dull and chalky, you may want to read How to Make Acrylics More Vibrant.

Watercolor paintings have clean brilliant colors because the pigments are mostly transparent. Watercolor paintings are often compared to the colors found in stain glass windows. It’s the transparency of the paint that makes a watercolors sing and the same technique can be applied to acrylics. Diluting acrylics with water will allow you to achieve a similar transparent effect.

Glazing can also be accomplished with oil paints but you have to wait for the paint to dry first before you can glaze over it. Acrylics dry fast so it’s possible to add numerous glazes to a  painting in one painting session.

Use Spatters to Simulate Textures

Fine spatters can simulate a variety of textures such as gravel, roads, and small leaves. It’s much easier to simulate these textures than it is to paint them by hand. They’ll also appear more spontaneous and natural.

An example of using spatters to create texture in an acrylic painting.

I used thin acrylic paint to create some subtle texture along the bottom of the painting to create the impression of gravel and pavement. I also used diluted acrylic paint to create a more fluid application of paint.

The paint has to be thinned to an ink like consistency in order to create fine spatters. Old discarded toothbrushes are often used to create spatters. Simply dip the bristles in the diluted paint and run your finger over them to get the paint to spatter. Wear gloves to protect your hands and only use the toothbrush for painting.*

Practice on a scrap of paper to get the feel for it. It’s easy to go overboard with spatters so try to restrain yourself. To protect important parts of the painting, lay pieces of paper over them. Make sure the surface of the painting is dry first.

If my spatters don’t work out, I wipe them off with a damp rag before it dries. Make sure the previous layers are completely dry before you try this technique. Otherwise the existing layers may smear. Some artists apply an isolation coat of gloss medium to protect the existing paint layers for this reason.

Acrylics that are made to be workable after they dry probably won’t work with this technique. If you were wipe the painting with a damp rag, it could disturb the previous layers. Always do some testing before trying out new techniques on an important piece!

Painting Fine Lines and Details

Fine lines are easier to paint with thin paint. Painting fine lines with heavy body acrylics straight from the tube results in lines with rough edges. Thick paint also requires a brush with stiffer bristles that make it easier to push the paint around.

Thinning the paint out with water allows it flow off the brush easier, producing sharper edges. Riggers or liner brushes are great for painting fine lines with thin paint. The soft and fine bristles work better with thin paints.

Fine lines painted with thick vs thin acrylic paint

The black line at top is Golden High Flow Acrylics, the bottom is Heavy Body Acrylics. The thinner High Flow Acrylics created a much sharper line. It was also easier to paint because the paint readily flows off of the brush.

Adding Too Much Water Can Weaken the Paint Film

While there are advantages to thinning acrylic paint with water, you should be careful not to thin it out too much.

When you dilute the paint, you’re also diluting the acrylic binder that is responsible for adhering the paint to the canvas. If you add too much water it may not adhere properly to the support. This also depends upon the absorbency of the surface that you’re working on.

Painting on a smooth, non absorbent surface requires a paint that has enough binder in it so that will form a solid paint film. A weakened paint film may have adhesion problems in the long run. Avoid over thinning your paints with water when working on these surfaces.

Absorbent surfaces such as raw canvas and paper will allow the paint to soak into the fibers which helps with adhesion. So if you’re using acrylic paints on watercolor paper you can probably get away with adding more water than the maximum recommendation of 25%. My post Painting on Paper with Acrylics has more information about which papers work best for acrylics.

The absorbency of primed canvases varies depending upon the quality of the gesso. The artist quality gesso made by the leading paint manufacturers is more absorbent than the gesso that’s used on the cheap economy canvases you’ll find at the craft stores.

If you find that you’re adding excessive amounts of water to your paint, then you should consider if you’re using the most suitable form of acrylics for your style.

Paint Viscosity

When manufacturers use the word “viscosity”, they’re referring to the consistency of the paint. “High viscosity” paint is thick heavy body acrylic. The low viscosity paints are thinner and don’t create as much texture. There’s an acrylic paint for every style.

Acrylic paint viscosity comparison.

Here’s an example of Ultramarine Blue in three different viscosities (thicknesses). On the left is High Flow acrylics which is similar to an ink like consistency. In the middle is Fluid Acrylics which has a consistency similar to heavy cream. Notice that it has a few streaks compared to the High Flow Acrylic. On the right is Heavy Body acrylic which retains brushstrokes and is much thicker than the other two. These paints happen to be from Golden, but other Manufacturers have similar lines of acrylic paint.

Heavy Body

Heavy body acrylics have a thickness that’s similar to oils. It’s great for creating impasto (thick texture) with a brush or palette knife. They’re too thick for watercolor techniques. You would have to add a lot of water to get it to flow like watercolor. Aside from weakening the paint film, the excessive water will make the colors look pale.

Soft Body Acrylic

Liquitex makes Soft Body Acrylics, and Golden has a line of Fluid Acrylics.  Both of these paints are thicker than ink but not as think as heavy body acrylics. They describe them as having the consistency of heavy cream. These paints are a good compromise because they have good coverage straight from the tube and yet they don’t require a lot of water to get them to flow. It’s also cheaper to stock one kind of paint instead of having to buy the same exact colors in different viscosities.

Acrylic Ink

Acrylic inks are very fluid and work well for drawing with dip pens and markers. Empty paint marker tubes are available for this purpose.

Acrylic ink can also be used on paper in a style similar to watercolors. The difference is you can paint over them when they’re dry without lifting the previous layers of paint. Liquitex makes acrylic ink and Golden calls their product “High Flow Acrylics”.

Use Mediums to Thin Acrylic Paint

Acrylic mediums are used to alter the properties of acrylic paint. They consist mostly of acrylic binder, so you can add as much to your paint as you desire without worrying about weakening the adhesion of the paint film.

Acrylic mediums are available in a variety of viscosities. If you just want to increase transparency without altering the consistency of the paint, then choose the acrylic medium that’s closest to the paint you’re using.

For example, gel mediums have the closest consistency to heavy body paints. So you could use them to make heavy body acrylics more transparent without making them thinner. Fluid gloss medium has a consistency similar to fluid acrylics. Golden Airbrush Extender is almost as thin as ink.

How to Thin Acrylic Paint Without Exceeding 25% Maximum

One problem with the recommended 25% maximum dilution is that it’s difficult to follow in a real world painting studio. When I’m working on a painting I don’t keep track of the percentage of water that I add to my color mixes. If I come back to a painting the following day, I’m definitely not going to remember.

I have a few recommendations for making this easier to follow. The first thing you can try is to use a very fluid painting medium to thin the paint instead of water. Golden Transparent Airbrush Extender is such a medium and it’s compatible with all of their acrylic paints. It has a very fluid consistency similar to ink so adding water may not even be necessary. You can add as much as you want to the paint.

Another approach is to dilute your favorite painting medium with 25% water and then add that to your paint. Then you can use as much as you of this diluted mixture as you wish without weakening the paint film. I talked to a representative from Liquitex and he agreed that this is an acceptable practice.

Conclusion

If you use an acrylic paint that has the consistency you prefer, then you may not have to dilute the paint with water. When you do add water, try not to add more than 25%. Absorbent surfaces will allow you to go beyond the recommended 25%. Remember to use distilled water if you’re planning on storing unused diluted paint for an extended period.

If you stick to these principles you can take advantage of thin acrylic paints, and your paintings should last a long time.

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