Some commonly asked questions about acrylic mediums are answered here.

From www.goldenpaints.com

Why are there so many Gels and Mediums?
Gels and Mediums are essentially thickened acrylic resins; the binder in acrylic paints. GOLDEN produces a large variety of them because they allow an artist to make their paints perform exactly how they want them to. The main differences in gels and mediums are: viscosity (thickness), rheology(feel), sheen, flexibility and texture. If you are having difficulties deciding which gel or medium to use, use these properties as a guide to figure out which product suits your needs. For example, if you want to thicken a Heavy Body Paint for impasto work, and desire a semi gloss sheen, then your choices are limited to gels thicker than the Heavy Body Paints, such as Heavy Gel, Extra Heavy Gel and High Solid Gel. Since there isn’t a High Solid Semi Gloss, you need to decide between the Heavy and Extra Heavy, and in this case adding Extra Heavy Semi Gloss Gel will thicken paint the fastest.

How do I know which Gel or Medium is the right one for me?
Because there are so many GOLDEN Gels and Mediums to choose from, an artist first needs to determine which characteristics they are looking to change, and which ones must remain. First, make a list of all of the positive attributes: What thickness is most desired? Do you want textures, such as brush strokes, or do you want to create a film completely level? Is the pigment load important? Do you want an opaque or transparent film? Is overall sheen of importance? Do I need a toothy surface? As you go through these issues, it is easy to eliminate many of the choices. However, modifying paints, gels and mediums can sometimes be very difficult, and it is always suggested to do tests and experiments on a small scale before doing applications on a canvas.

What’s the difference between a Gel, a Medium and an Additive?
In general, gels and mediums form a film, while additives do not. Additives, such as GOLDEN Retarder, Acrylic Flow Release and water, may change the working properties of an acrylic product, but do not change the paint layer properties. They do not have any binder in them and an artist needs to be concerned with how many additives are in their paint mixtures. Too much Retarder results in a slow drying, tacky, and sometimes uneven film. Too much Acrylic Flow Release has similar side effects, as well as creating a lot of undesired foam (trapped air). Too much water can result in poor film formation, as the acrylic binders are spread out too far to form a continuous film. The difference between Gels and Mediums is a little tougher to determine. Generally, a Medium is pourable, and gels are not. However, there are exceptions to the rules. For example, Clear Tar Gel, is pourable, but is named a “Gel”. Certain products, which use the general description of gel or medium, are normally assessed by true viscosity (thickness or resistance). Most artists lack the equipment needed to determine actual viscosity, so at Golden Artist Colors, we are left to decide this for ourselves.

What are all of your Gels and Mediums used for?
For the most detailed information refer to the GOLDEN Information Product Guide and other literature. We manufacture many different Gels and Mediums that all have specific uses. Almost all can be used to “extend” the paint, which means you can get more paint by adding a Gel or Medium. This can be done to the point where the artist can make “glazes”, or very transparent, colored layers. There are also products that alter the sheen, as well as those, which can make the paint harder, better leveling, etc. These products come in a wide array of thickness, ranging from the thinnest Airbrush Medium to our Extra Heavy Gels.

From www.goldenpaints.com