There are many different types of gels, mediums, and additives that can be combined with acrylic paint. These different additives can greatly change the properties of the paint that you are working with. Whether you want to add texture, thickness, or extend the drying time of acrylics, there are many different products available.
Polymer medium are usually slightly thinner then standard acrylic paint. They allow you to extend the paint as well as create glazes. Mediums can even be poured on top of a canvas that is lying down to create all kinds of unique effects. Polymer mediums usually come in matte and gloss varieties. Gloss dries to a transparent sheen. As the name implies, matte medium dries to a more matte finish. Usually mediums will appear milky white when they are wet, but dry totally clear. This can make it difficult to judge what color the mixture will be when it dries. Often colors will appear lighter then they will be when the dry. So the best results will be achieved by practising with different colors, and different concentrations of paint to see what the end results will look like.
Gel mediums are similar to polymer mediums except rather then being thinner than heavy body paints, they are usually thicker. Acrylic gel mediums are useful for creating textures on a painting. They come in different degrees of “heaviness”. Heaviness is the amount that they will hold their shape once they are applied to a surface. Heavy body gels will hold sharp peaks and marks. Lighter bodied gels will slump to a certain extent with gravity. This is especially true of products that are labelled “self-levelling” as they are designed to slump to an even hight. Just like mediums, gels are usually milky white when they are wet, but then dry clear. They can also be used as a type of glue to attach collaged elements to an artwork. Gel can also be applied with a brush to create bold impasto brush marks.
Molding paste is a variation of gel medium. It has an even heavier body then gel. It can be built up almost like a sculptural material. Molding paste usually doesn’t dry as clear a regular gel. This means it would be difficult to create transparent effects with it. Also, its whiteness can tint the paint a lighter color.
Pouring mediums a more liquid version of polymer mediums. They are used to create interesting pouring effects. This is often done by laying the canvas or panel flat on the ground then pouring the mediums over top. While this could theoretically be done with regular polymer medium, there is one problem: crazing. Crazing occurs when the surface layer of the paint dries before the bottom layers. As the surface layer dries, it shrinks. This shrinking of the “skin” of the paint makes it crack and rip. Some artists actually use this property of paint to their advantage, as it lets them create truly unique effects. Pouring medium has special properties that reduce the likelihood of it crazing. Although some crazing is still possible, especially if you are using it in a very dry environment. They are best mixed with fluid acrylics (acrylic paint with a thin liquid consistency, but still having a high concentration of pigments) because it will be easier to combine the two. If mixed with heavy body acrylics you can run into problems with the heavy body acrylic not fully mixing with the medium. This can result in a lower tint strength as well as clumps of heavy body paint in the fluid mix.
Crackling mediums are the complete opposite of pouring mediums. They are designed to craze and create a crackle effect. This is almost the type of effect you would see on an antique glazed vase.
There are many different acrylic gels and mediums with added substances that can create interesting textures. Pumice stone is one type of additive that can create a rough, concrete-like surface. Other additives include glass beads.
Retarder additives are used to slow the drying time of acrylic paints. The normally quick drying time of acrylics is one of its greatest strengths, but it can sometimes be an unwanted property. It is great because it allows the painting to dry quickly, so you can add other layers or paint over a mistake. On the other hand if you are trying to blend, model, or make subtle shifts in color, they quick drying property, can mean your paint dries before you want it too–both on the painting and the palette. Retarders can be added to the paint to prevent this.
Glazing is a technique that has been used for hundreds of years by painters. It works by layering semi-transparent layers of color on top of each other. The advantage of using glazes is that the colors mix optically rather then physically. This means if you wanted to make green you would paint a layer of yellow, then apply a thin glaze of blue over top. The light would reflect from the surface of the canvas, through the glaze and back to your eye, so the paint would appear green. Advocates of glazing say that this makes for more luminous colors. Glazing is traditionally done with oil paints, but glazing mediums allow you to apply thin, semi-transparent layers of color in the same way. A big advantage of using acrylics over oils is that it dries quicker then oils. That means you won’t have to wait as long between layers of glaze. Acrylic glazes can also dry clearer then glazes made with some oil paints, which can take on a slight yellow tinge.
Adhesion mediums like Golden Gac 200 can be added to regular polymer mediums or gels to increase their adhesive properties. This is useful if you are making a collage or laminating a canvas to a panel.
Stiffening mediums like Golden GAC 400, strengthen and stiffen fabric fibres. This means that if it is applied to a canvas, the canvas will be more rigid and stiff.
These are some of the common mediums and additives used by acrylic painters. Selecting the right one can allow you to create all kinds of different techniques.