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 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)
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.
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)
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.
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)
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.
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)
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 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)
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.
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)
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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Hi Nancy! Thank you for this great information. I’m painting on aluminum foil That has been adheres to stretched canvas (taught, but a bit of a give). One video/artist says to just use matte medium to prep the aluminum. Another says to use GAC 200. I’d feel better using GAC, but so you feel that’s the correct choice?
I understand your question, but want to preface my answer by saying that commercial aluminum foil may not be archival for fine art use. It may have coatings on it, or something else meant for cooking purposes, not art. However, you can always test it on something you don’t care about before applying your process and materials onto something more precious. Perhaps, though, you meant aluminum (imitation silver) leaf. If that is the case then this may be more appropriate for fine art use. Now to answer your question. Matte medium will probably peel off of the metal. GAC 200 is a special medium meant to adhere to metal. If you clean the metal with a rag and denatured alcohol first, then apply the GAC 200 that should be fine. I’m assuming when you say you want to prep the aluminum you mean that you want to apply something to help adhere applications of paint over the metal. Just a tip. Applying GAC200 is VERY tricky. It will not cure properly if used in a space that has a temperature cooler than 70 degrees Farenheit. You only need it this warm while applying it. Then it’s OK if the temperature drops, but all acrylic needs to be above 56 degrees Farenheit during its first 2 weeks of drying to cure properly. If you mix the GAC200 with GAC500 in a 1:1 mixture, you will find it easier to apply. Whether you apply it by itself or in this mixture, do not overbrush. Once it starts to get tacky (within seconds sometimes) if you brush over it you will create microscopic bubbles that create a whitish film that cannot be reversed. Apply the GAC200 (or mixture with 500) very thinly, without diluting it, but instead brushing it out smoothly. I apply it in small squares about 6″ x 6″ at a time so I can smooth it out before it gets tacky.
Hi I wanted to buy gac 800 and made and error and got gac 400 I do pouring paint and sometime paint with a brush what can I do with the gac 400 please help
I’m sorry your made a purchase error. The GAC400 is meant to be used with fabric to make it stiff. I used it to create masks by brushing it onto fabric, then while wet placed the fabric into plastic molds. When dry I could remove the fabric that took on the shape of the mold.
I honestly recommend you do NOT use it for pouring or with a brush. You can always experiment to see what these processes will do with the medium, but these specialty mediums are really made to work successfully with one process. By trying to use the medium with a purpose it is not meant for could actually cause you to waste more products. Just saying…..
Hi Nancy! I am planning to use kolner burnishing clay over my acrylic designs that are made with a mixture of Grunbacher molding paste and golden gloss medium. I know that acrylic paint surfaces are porous, but what about molding paste and gloss medium? The Kolner burnishing clay should be used on a nonporous surface, or porous surface sealed with something, like shellac or primer-p. I don’t know what primer-p is. I was wondering if I paint a layer of gloss medium and then apply 2+ layers of GAC 200 to seal the molding paste mixture would that provide an equivalent adhesion? I am using burnishing clay because I want my gold surface to have that metallic sheen achieved by burnishing over the matt that comes naturally. I already have some Golden gloss medium and GAC 200. Please advise me what you think is best. Thank You!!!
Wow you are describing quite a process! First let’s define porous and non-porous. Anything that is glossy is non-porous (think plastic wrap) and anything matte is porous (think paper towels). So when you say that acrylic paint surfaces are porous, I think that might be confusing you. Acrylic paints and mediums vary in this respect. Molding Paste is actually satin, so it is porous. Gloss Medium is glossy so it is non-porous.
I have not heard of primer-p either, but Gesso is a primer and is usually matte so it is porous. The cheaper Gessoes don’t have as much pigment in them and may be glossier, so less porous. You mention applying Gloss Medium, then GAC200 over the Molding Paste. With this process you are basically sealing a non-porous surface (the Molding Paste) with a porous layer (Gloss Medium and GAC200). I think you could apply the clay right over the Molding Paste and it should stick well. I do suggest experimenting on something you don’t care about to make sure this works. Also I am used to using Golden’s Molding Paste, which is porous, but if you are using another brand you would have to test it.
If you ever have a glossy surface, you can lightly sand it to change it to porous, or apply a porous product. My favorite porous product is Golden’s Pastel Ground. I rarely use it plain, right out of the container, as it is thick and gray. But when I dilute it heavily (at least 60% water) it makes a clear grit that is very porous. Hope this helps!
Hi Nancy, first of all, huge thanks for putting so much useful information out there. It’s a very tricky topic and I am so glad to have found your video. I have a bit of a random question…can I use charcoal over gold leaf once it has been sealed with MSA gloss varnish and a coat of GAC 200? I don’t know why I would do this, just to experiment perhaps because I use charcoal powder thinned down with alcohol, and I am curious to see what the gold leaf looks like through the transparent layer…somewhat similar to transparent oils over gold leaf. Would love to hear your thoughts on this please?
Also, if planning to use oil paint over gold leaf, can I seal with colorless shellac instead of GAC 200 over the MSA varnish? I saw this briefly mentioned somewhere, but not discussed in detail.
Yes you can use charcoal or any other drawing medium over gold leaf once it has been sealed. There is a trick though, because charcoal and other drawing materials require a toothy surface to show up (unlike markers and oil pastels). So after the leaf is sealed, the surface is glossy and the charcoal won’t be able to grip the surface. You need to add a gritty surface that is clear. This is tricky because any type of acrylic matte medium contains a fine white powder to make them matte. This will change your glossy gold surface and dull it permanently.
There is only one product that I know of that will work well. I use it for this purpose and many others all the time. I use Golden’s Pastel Ground. Now that product is very opaque and gray unless you dilute it enough. If you add about 90% water to it (little by little so it mixes smoothly and doesn’t get lumpy) then you can brush apply it over the sealed leaf, and it will add a clear grit over the leaf. Let it dry for a day. This surface will now accept pencils, charcoal and other materials really well. You can even use watercolor washes on it.
The big thing I want you to remember is that it will look like it dulled the leaf after it dries. But after you finish applying your charcoal, if you spray it with any gloss medium (that can go in a spray) the original look of the gold will come back. I suggest experimenting first with something you don’t care about. If you don’t add enough water it will leave a dull film over the leaf after the gloss is applied. If you add too much water you won’t have a good grit. Always err on the side of adding too much water because you can always apply another coat of the pastel ground wash for more grit, but you can’t remove a layer of it if you didn’t add enough water. To apply the gloss layer after drawing, I suggested spraying because charcoal is so delicate. When I use pencil I just brush apply a gloss medium over it to seal the pencil and bring back the gold shine. If you are careful enough and the charcoal isn’t too heavily applied you can try brush applying the gloss.
Another option that may be easier is to lightly sand the sealed leaf surface. This may give it enough grit to hold the charcoal. You will still need to seal the charcoal as above so it doesn’t keep coming off.
If you are using oil paint over leaf, you can seal it with any clear sealer meant to go over metal. I stay away from shellac because it will add a strong yellow color over the leaf, and also I do not know enough about it to recommend it go in between metal and oil paint. Actually you can use any clear glossy spray from Home Depot or Walmart or any paint store. Anything in a spray can is usually solvent based. This will seal the leaf, then you can skip the GAC200 and just work with the oil paint.
Nancy, thanks so much. I actually might have the pastel ground, so this is a perfect workaround! I also have the absorbent ground although I don’t know if it is of much use here?
I just want to clarify, I usually spray a fixative after my charcoal drawing….can I still do that? I use the Krylon workable fixatif. And if I have the Golden MSA gloss varnish, can that be sprayed at the end of the charcoal drawing…is it the same as what you refer to as a “gloss layer”?
Thanks so much!
Great! Glad you already have the Pastel Ground. Diluted as I described is a great way to get so many interesting effects, especially if you want to add drawing materials over an acrylic painting surface, or metal leaf. The Absorbent Ground is a lovely ground but very opaque and will not get transparent when diluted like the Pastel Ground. Use this over a surface to create a good ground for watercolor, or other types of washes like acrylic with lots of water, or oil paint with lots of turps.
I think the fixative you mentioned will be OK to spray over your charcoal drawing. As usual, experiment first on something you don’t care about to test it in the way you want to use it. The Golden MSA is the exact same thing as the Archival Varnish in a spray can. The reason it has a different name is because it contains an additional ingredient to allow it to go easily through a spray nozzle. You can definitely use the MSA in your own spray equipment, like an air gun. The directions on the can say to add more solvent if using it as a spray then you would if brush applying it. This will give it a gloss layer, but remember the MSA and Archival Varnish are both removable, so I recommend sealing over it with the GAC200 if you plan to continue working over it. The MSA and Archival Varnish are meant as a final topcoat. Being removable means it allows for the painting to be cleaned if ever needed.
Hi, I am painting wood blocks and sealed them with gloss medium. They are still tacky and thought about using a coat or two of gac 200 to harden them. Do I need a varnish as well?
If you applied the gloss medium thickly then it may take awhile for it to dry. In the future you may want to dilute the medium slightly so it sink into the raw wood, without sitting on top where it can get tacky. If it is still tacky then it may not be dry yet. If it is humid, warm or less than 3 days since you applied the medium, it may need more time to fully dry. Then on the other hand, warm weather can make the gloss medium feel tacky even if it is dry. If you apply a matte medium over the tacky gloss, it may keep it from getting tacky in the future. Varnish is only needed over the paint if you want extra protection. Fine art varnishes have UV protection that can keep colors from fading if you feel this would be an addition to your work.
I am a new artist learning to paint using professional-quality Acrylics – Liquitex or Golden. What GAC # would be suitable for this purpose?
The GAC mediums are for special purposes. You actually don’t need any GAC mediums as a new artist learning acrylic. The GAC100 is your basic medium that is specially formulated to be very thin. I use this to coat wood panels, and if I want to thin a thicker paint without making it more transparent. You can use this if you don’t like texture or brushstrokes as your main medium. Otherwise I suggest using Acrylic Gloss Medium instead of any of the GACs as your basic medium. I hope this helps answer your question. If not, please give me more details about what effects you are trying to achieve and I can offer more suggestions.
Hi Nancy, Great information on your website! I’d like to thin Golden Molding Paste so it can be applied with a thin-tipped applicator. Which medium do you recommend? I’ll be applying it to a painting on a wood panel (panel has been sealed with GAC 100). TIA, Ann (oil painter)
I’m curious as to why you would want to thin the Molding Paste, which is made thick on purpose to create a texture. You can thin it by adding GAC100 (a thin acrylic medium) or adding water, but wouldn’t it be easier to use something else that is already thinner to go through your applicator? What about adding white paint to a Soft Gel instead? Just a suggestion. If you send me more details about your technique I may be able to offer more ideas.
Hi, I’ve made my own inks using natural products, I wondered if the GAC 100, would make the inks suitable for use with canvas on a fairly large scale….. many thanks Sacha
I suggest you contact Golden’s tech department for the best answer. When you say “natural products” that doesn’t give me enough information to know if your home made inks are compatible with acrylic. I do know that creating stable and reliable acrylic paints involves a lengthy process, unlike oil paints and watercolor, acrylic needs more science to stick well to surfaces and adhere for a lengthy time. I would not recommend adding GAC 100 to home made inks if you expect it to be permanent and to create a good paint. Hope this helps!
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!
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.
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
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?
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
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!
Thank you for the comprehensive descriptions for each of the GACs. I learned a lot today.
I’m happy to hear from you that you learned from my descriptions of the GAC’s. They can get confusing.
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?”
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.
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?
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
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?
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!
i use a Golden Air brush medium to create washes – can i use Gac 500 for this purpose
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.