Special Acrylic Painting Mediums – Tips to use GACs

by | Apr 25, 2020 | Blog | 16 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. Cynthia Baumhauer

    Cynthia Baumhauer November 10, 2021
    Nancy, I think I have the answer to my previous question from reading your past replies.

    I can apply 4 coats of GAC200 to the leaf portions of my piece then apply a water-based varnish that is either matte or gloss.

    Thanks so much for this blog!

    • Nancy Reyner

      Hi Cynthia,
      Yes you have the right idea! If you want to apply a water-based varnish as a last coat on your piece, you can, but only one coat of the GAC200 may not be enough protection from the ammonia in the water-based varnish. I recommend your idea of 4 coats. Let it dry at least a few days. Apply the water-based varnish thinly so it doesn’t puddle up. I suggest experimenting on a piece you don’t care about to make sure this process works. The important concept is to add enough coatings of the GAC200 so that the leaf is protected from tarnishing due to contact with ammonia.

  2. Cynthia Baumhauer on Nov. 9, 2021

    Nancy, I watched your youtube video on applying gold leaf. First time using the leaf, I applied some silver & gold leaf (not solid on a non-porous surface). I then painted w/acrylic covering most of the leaf, but letting some show through. I then applied GAC200 over the entire piece (maybe a mistake?). After one coat of GAC200 I realize the piece is way too glossy. Can I now apply a water-based matte varnish over the entire piece? It’s mostly acrylic, not much leaf showing. Or what remedy would you recommend to cut the gloss?

    I am anxiously awaiting your book, Acrylic Illuminations which I have recently ordered!! Thanks, Cynthia Baumhauer

  3. Swati K

    Hello Nancy, I had a quick question. I had used gold flakes on my trial fluid art painting and pasted those goldflakes with mod pondge. After 3 days, I sealed it again with mod podge but it seems to turn black. I find Gac 200 too expensive for my budget. Could I use Gac 500 and if I use Gac 500, will I have to do a Msa varnish on it?

  4. Tanja

    Hi Nancy,

    I have a paper screen.the paper itself is thick and I would like to do oil gilding on it.
    Which gac would you suggest to use to prime and seal the paper. I want to avoid the oil size breaking down my paper.
    I have the 800 and the 200 for plexiglass and glass
    I think you would suggest the gac 100

    • Nancy Reyner

      Hi Tanja,

      It’s good you are planning to seal the paper to apply oil-based adhesive for your gilding. Just to be clear, you mention “oil gilding”. I am curious what you mean by that, am thinking you mean you are using the oil-based adhesive to apply leaf, and wondering why you are choosing to use the toxic adhesive? I’m sure you have a good reason – just curious. If you used the water-based adhesive it is non-toxic and will adhere the leaf very well. You can they apply oil paint over the leaf once it has dried from the application.

      Now back to your question. It is important to seal your paper prior to applying ANY leaf adhesive. If you don’t the adhesive will absorb quickly into the paper, and then it won’t be able to hold the leaf. Sealing the paper is a must. It will ensure good adhesion of your leaf, and then if you use the oil-based adhesive it will, as you are thinking, keep the paper from damage due to the oil.

      So now, which product to use to seal paper. Any gloss medium is a sealer. Matt and satin products absorb and are not sealers. You can use any regular Gloss Acrylic Medium. I would not use specialty mediums like pouring mediums (that’s the GAC 800) or gels. The GAC 200 is tricky to use because it dries super fast and can create issues if you brush over it while it is drying. The GAC 100, as you mentioned, will work but is very thin so you may need two coats. You can probably get away with just one coat if you use a regular acrylic medium.

      Hope this helps!


  5. Estella

    Thank you for the comprehensive descriptions for each of the GACs. I learned a lot today.

    • Nancy Reyner

      Hi Estella,
      I’m happy to hear from you that you learned from my descriptions of the GAC’s. They can get confusing.

  6. Erik

    An awesomely comprehensive dissertation on an area of knowledge too rarely addressed online! I have, personally, benefitted from your sharing of your deep expertise, and now know precisely which GAC product I need for an upcoming project in acrylics. Kudos on this article. However, regarding your preventative use of GAC 100…no offense or snooty-Art-know-it-all-ness intended, but, don’t you mean “Substrate-Induced Discoloration?”

    • Nancy Reyner

      Hi Erik,
      I’m glad to hear from you and that this article was helpful to you. Thank you for alerting me to check out the acronym SID. I just checked it out on Golden’s website and it looks like I stand correct. SID stands does for Support Induced Discoloration. They like to use the term support for substrate but they do mean similar things.
      Thanks again!

  7. Lorraine Bohn

    I am sealing cradle boards for painting on. I was using GAC 100 but ran out, can I use GAC 800 instead. I am not close to any art stores. Any other suggestions?

    • Nancy Reyner

      Hi Lorraine,
      Instead of the GAC100 to seal wood panels, you can definitely substitute by using any REGULAR acrylic gloss medium. I absolutely do not recommend using the specialty medium GAC800. That particular medium is meant for pouring, not for brush application, and has some other qualities that may not result in a good application on wood for sealing purposes. Hobby stores should carry a regular gloss acrylic medium, and certainly you can purchase it easily online. Here is an article I wrote with more information on sealing wood panels ​How to Prepare Wood Panels for Painting

  8. 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?

    • Nancy Reyner

      Hi Angela,
      It’s a good idea to go non-toxic when you have kids around. Once you apply the leaf over your acrylic painting you would, as you said, want to seal it with something non-toxic. The only thing that will work is GAC200. This is very tricky to use because it has ammonia in it while it is wet, so you want to apply it thinly so it dries so fast the ammonia dissipates before it has time to tarnish the leaf. So don’t add water to the GAC200, don’t have water on your brush, don’t apply it thickly, apply it to small areas at a time so you can quickly spread it thinly. Use a smooth soft bristle brush. Do not overbrush. What I mean by this is that if you brush over it once it has started to tack up (usually within seconds) it will create microscopic bubbles that will dry in bubble form creating a white film that cannot be removed. I suggest practicing applying the GAC200 on something you don’t care about before applying it on your finished painting. You are correct that you can apply a water-based varnish (matte or gloss – your choice) over the acrylic painting parts, while applying the GAC200 over the leaf. As an alternative to your process you described, you can apply four coats of the GAC200 over the leaf, then apply a water-based varnish over the entire painting, acrylic painted areas and leaf, all at once. Hope this answers your question!

  9. 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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Professional fine artist Nancy Reyner created this blog about art, painting and creativity from her career of over 30 years. She lives in Santa Fe, New Mexico. Subscribe below for free tips on art and painting.

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