Special Acrylic Painting Mediums – Tips to use GACs

by | Apr 25, 2020 | Blog | 3 comments

Feel confused by all those GAC mediums on the art store shelves? What ARE they all for?


I am happy to share what I’ve learned through many years working as a Certified Working Artist for GOLDEN, and how I use these mediums in my paintings.

First things first! What does GAC stand for? This acronym stands for GOLDEN’s official company name GOLDEN ARTIST COLORS. The GAC is used in the product names for six Special Purpose Mediums.

Each GAC medium is specially formulated for a specific purpose. These mediums all have one thing in common – they are all thin in consistency. Being thin doesn’t mean they are diluted with water or any other extender. Instead these mediums use a polymer that is already thin and not thickened. Acrylic is actually naturally thin, so most of the thick acrylic products, like heavy body paints and thick gels, go through a thickening process in production. Fluid acrylic paints, fluid acrylic mediums and all the GAC mediums do not get thickened. 


GAC Special Purpose Mediums from GOLDEN

Under each of the six GAC mediums in this list, I first give GOLDEN’s description gathered from their website. Following that, I add my hopefully user-friendly translations, along with how I use it for painting purposes.

GOLDEN’s description of their GAC Specialty Mediums
Specialty Acrylic Polymers are based on 100% acrylic polymer emulsions. The GACs are useful as mediums or modifiers of acrylic paints. GACs can be used to extend acrylic colors, to regulate transparency, create glazes, increase gloss, reduce viscosity or improve adhesion and film integrity. GAC polymers can also be used for binding pigment solids for various effects and surfaces. Unlike the other GOLDEN Mediums and Gels, GAC Acrylic Polymers have only a minimum amount of thickeners, levelers, defoamers and surfactants to ensure good film formation.

The consistency of the GAC polymers is more fluid and thin than other GOLDEN Mediums, so they will reduce the thickness of most GOLDEN Acrylic Paints. GOLDEN Fluid Acrylics are slightly thicker than the GACs, but will exhibit less change in viscosity with the addition of a GAC polymer.

Each GAC is a unique polymer with unique benefits and applications. Refer to the product descriptions below to find the proper medium for a particular applications.

For health and safety information, download the Safety Data Sheet.


GAC 100

GOLDEN’s description
GAC 100 is a thin, translucent, gloss medium. Useful for thinning or extending colors as well as increasing flexibility. Wets out solids more readily than other polymers and is useful for creating homemade paints. GAC 100 provides basic protection from Support Induced Discoloration (SID), for better protection see GOLDEN Gloss Medium. (Golden’s Item# 3910)

My comments
Consider this as both a stain sealer as well as a standard use thin medium. To use as a stain sealer apply on raw unsealed surfaces – wood, canvas, paper – to prevent staining (staining is a yellowing of subsequent layers of acrylic, that happens when water-soluble impurities from an unsealed seep into the acrylic paint layers).

To use to thin paints, just mix into fluid paints to make the color more transparent without thickening the paint, and mix into thicker heavy body paints to make more transparent, or to decrease the paint’s thickness for smoother applications. A great alternative to adding water to paints to thin, which will dilute the paint, changing its sheen and decreasing color intensity.

I brush apply this product in one or two layers to all my canvases and wood panels (any surface I will be painting on) directly onto the raw surfaces. This layer or layers will inhibit what is called SID (Stain Induced Discoloration). Without this layer, you may see a slight yellow staining on light colors and pastes, where water soluble impurities move through layers of acrylic. Without this stain sealer, the more acrylic layers you apply, and/or the thicker the layers, the faster and more visible the yellow stain will appear. Staining happens usually within 20 minutes of applying an acrylic layer if the surface is not sealed. After this product is applied to the painting surface, I then apply primer (Gesso). I also use this as a thin medium to thin my paints without adding water.


GAC 200

GOLDEN’s description
Hard acrylic extender for non-porous surfaces is the hardest and least flexible polymer we offer. It is Ideal for mixing with acrylic colors to increase film hardness, reduce dry film tack and to increase adhesion to many non-porous surfaces. Dries to a clear, high gloss, finish. GAC 200 is not recommended for flexible supports. (Item# 3920)

My comments
Enhance adhesion.
Use on non-absorbent rigid surfaces (such as metal or glass) before applying paint to help adhesion between paint and surface.

Mix in with paints to make them harder, for use outdoors and/or on non-absorbent rigid surfaces. Do NOT use on flexible surfaces like canvas. Must use with temperatures over 70 degrees for first 24 hours of drying.

I use this to mix into my paints, or as a layer by itself, to add extra strength to adhesion between paint and surface. It is useful for rigid surfaces like wood panel, glass and metal. It is not as flexible as other acrylic mediums, and is not to be used on canvas unless it is mixed 1:1 with GAC500.


GAC 400

GOLDEN’s description
Stiffens natural fibers and fabrics and is useful for stiffening unprimed canvas or sculpting and shaping fabric. Fibers saturated with GAC 400 will dry to a hard, stiff film. (Item# 3940)

My comments
Use it to stiffen fabric. I used it with great success one time when I taught an art workshop for families at a museum. They coated pieces of fabric with this medium, then draped the wet fabric over plastic mask forms. When the medium dried, the fabric was stiff. The plastic mask form was removed and the fabric kept the shape of the mask, which was then decorated with paint, feathers and glue guns, etc. This was perfect for children and non-artists as they could finish their mask within the short workshop time frame.


GAC 500

GOLDEN’s description
Gloss extender for Fluid Acrylic Colors is a unique balance of film hardness and flexibility offering increased leveling, increased mar resistance and decreased dry film tack. GAC 500 is particularly useful for extending Fluid Acrylic Colors with minimal property change. It can be mixed with Airbrush Transparent Extender for a fast-drying, sprayable isolation coat. (Item# 3950)

My comments
A hard but still flexible acrylic. Use like GAC200 but on flexible surfaces. Use for taping hard edges. Apply at tape edge so paint won’t seep through. A hard but still flexible acrylic. Use like GAC200 but on flexible surfaces. Great for taping (apply to edge of tape, then apply paint to keep taped edges sharp).


GAC 800

GOLDEN’s description
GAC 800 is a low-crazing extender for pouring acrylic colors. “Crazing” is the formation crevices in surfaces that develop as acrylic paints and mediums dry. The addition of GAC 800 promotes drying with a smooth, even film, good gloss and flexibility, but with moderate clarity. GAC 800 is also useful for adhesion to chalky surfaces. (Item# 3980)

My comments

Pouring medium can be poured very thickly without crevicing. Pour as is, or add color for a colored pour. Pour to smooth out texture My main “go-to” pouring medium.


GAC 900

GOLDEN’s description

Heatset fabric painting medium, offers a very soft hand and laundering stability. Mix with High Flow Acrylics to produce “tie-dye” effects, or blend with GOLDEN Heavy Body, Matte or Fluid Acrylics for brush or screen application. For more information on fabric painting see this Application Information Sheet (Item# 3990)

My comments

Use when you want to paint on wearable fabric. Add to paint, or apply to fabric. Use a dryer to heat seal. Then the paint will be washable in the laundry.


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  1. Angela

    Hi, I’m looking for tips on my painting where I have a few sections where I put imitation gold leaf on top of acrylic paint. I want to varnish it for protection, bit I’d like to avoid MSA varnish b/c I have kids, so will try GAC200. I would like to keep the gold leaf parts shiny but have the rest of the acrylic paint more matte. I am thinking I would have to manually brush on matte varnish on the paint parts, and brush GAC 200 on the gold parts. Any other tips?

  2. jesse colvin

    i use a Golden Air brush medium to create washes – can i use Gac 500 for this purpose

    • Nancy Reyner

      Hi Jesse, I like to use the terms wash and glaze to designate two completely separate ways of painting. A wash uses water to heavily dilute the paint. This breaks down the binder allowing the watery paint to sink into an absorbent surface or puddle up on a non-absorbent surface. I use washes to create certain effects unobtainable using mediums. A glaze on the other hand uses mediums not water to change the paint. This sounds like what you are doing with the airbrush medium. Airbrush medium needs to be slow drying so it won’t clog an airgun. It uses a thin polymer much like the GAC500. I suggest you experiment and try using the GAC500 for your purposes. They will be similar in consistency – both being thin – and clarity as both dry clear. However there is a difference in drying times. The GAC500 will dry much gaster than your airbrush medium. I use a mixture 1:1 of airbrush medium to GAC500 for a spray application of a clear coat also called an isolation coat. So go ahead and try it out as there is no harm in it for your purposes. Again, if you are working with an airbrush the GAC500 alone will dry so fast it will clog your sprayer.

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About Nancy

Nancy Reyner is a professional fine-art painter with over 30 years experience using a variety of mediums including oil, acrylic, watercolor and mixed media. She has appeared on television for HGTV’s “That’s Clever,” and authored several best-selling painting books with F&W Media. She currently lives in Santa Fe, NM. Read more.
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