There are so many additives and acrylic painting mediums available, it can get confusing as to what does what.
And just to further confuse things, remember that in art, there's two meanings of the word 'Medium'.
It can describe the type of paint you're using such as oils or acrylics. But in the context we're looking at here, it means an additive to your acrylic paints.
Here's a resume of the most common acrylic mediums and a brief description of each...
Acrylic paint's biggest asset is its speed of drying. This is also its major drawback if you're trying to blend colors on the canvas or you're just not the fastest painter in the world. Retarder slows the drying speed by up to 50%. Mix it with the color before applying it. Using more of it with paint increases transparency. However don't use it to excess in a mix or the paint won't dry at all! Having said that it's probably going to be the most used of your acrylic mediums.
Used neat to seal semi-absorbent paint surfaces like unprimed canvas, for both acrylic and oil painting. Produces matt white surface with a slight texture or tooth. Can be obtained in black as well.
Colorless liquid added to colors to reduce viscocity and enhance flow. Good for large areas of work where smooth finish is required or straight edges in conjunction with masking tape. Add bit by bit to the color until you have the right consistency.
Available in gloss or matt finish. Increases transparency of colors for glazing work. Glazing is where a translucent color is painted over another dried color to let that underlying color glow through eg: skin tones.
Impasto Gel Medium
A heavy gel which, when added to paint creates a thick, buttery paint which only loses slight opacity. Bulks out the paint to permit coverage of large areas of thick (impasto) color. Brush and knife marks will be retained whilst drying time is slowed slightly. Available in matt or gloss.
A thick, white paste which is used prior to applying paint. For this reason, one of the most used acrylic mediums, because it's more economical than using thick dollops of paint. Depending on thickness used, it may crack. However, if used in conjunction with Impasto gel, cracks can be filled in. A combination mix of these two acrylic mediums, painted over, will work well on flexible surfaces like stretched canvas.
Pearlescent/Metallic Tinting Medium
There are a variety of products available which will add a pearlescent or metallic shimmer to colors. Used alone, they still provide a translucent pale colored effect, the actual tint depending upon manufacturer.
Similar to pearlescent medium, these are best used on dark or black surfaces to get a shimmering metallic effect. When mixed with another color, the color changes depending which angle it is viewed from.
Acrylic Matt & Gloss Varnishes
Now we come on to acrylic mediums which are painted over the top of your picture as opposed to being mixed with the paint. These will protect dry acrylic paintings from dust and most environmental pollution. Can be obtained as a liquid for b ush application or in a spray format.
However, be careful!
The term 'varnish' tends to be used loosely for any clear, protective coating on a picture. Some acrylic 'varnishes' are spirit-based and can be cleaned off at any time in years to come with low odor thinners or turpentine. The picture can then be re-coated with clean varnish.
Many acrylic 'varnishes' however are water-based and made from an acrylic polymer, similar to acrylic paints. Once dry, they cannot be removed. Like their spirit-based counterparts they resist blooming, cracking and yellowing if used correctly.
However, if circumstances arose in the future where you wanted to replace this type of coating, you've got a problem. So when using a varnish, always check out beforehand whether it can be removed and whether this is likely to be an issue for you.
Getting started with acrylic paints
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Painting with acrylics can be great fun. However, the acrylic paint brushes you use are essential to the quality of your finished painting.