How Much Water Can You Safely Add To Acrylic Paint?

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A test panel showing High Flow Indigo Blue (Anthraquinone) and Fluid Anthraquinone Blue mixed 1:20 and 1:100 with a blend of 1 part GOLDEN Acrylic Medium and either 5 or 10 parts water.

In blog posts and workshops the warnings can seem dire: add too much water, we are told, and the acrylic binder will break down, causing paint to flake off or adhesion to fail. Some will set the magical mark at 30%, others at 50, but almost universally the rules are presented without citing or showing any evidence. So we thought we would fill in some of those blanks and share what our testing reveals.

Water is clearly a central ingredient of artists’ acrylics, which even at the start will usually contain somewhere between 45-55% water. So it can feel natural to be hesitant before adding a lot more. Surely there must be a breaking point, but where? For years our standard advice was that a 1:1 ratio was very safe for most of our paints and mediums; plus, it had the advantage of being easy to remember while greatly erring on the side of caution. However, our current testing shows you can go a lot further than that before encountering significant issues. Just how far? We think you will be surprised.

Core Findings

Adhesion: We saw no adhesion failure of any of our paints, no matter how thinned down with water, when applied on top of acrylic gesso.
Sensitivity to water and other acrylic products: Colors we tested that are typical of our line, like Yellow Oxide and Phthalo Blue Green Shade, showed no sensitivity to water or various acrylic products even when thinned with 20 parts water. In fact, sensitivity was mostly limited to worst-case scenarios of highly thinned down mixtures (1:20 and 1:100) involving pigments like Raw Umber, which has a high clay content, or the more dye-like Anthraquinone Blue.
A safe way to thin any ratio of paint to water: By using a minimum blend of 1 part acrylic medium to 10 parts water, we essentially eliminated sensitivity to water or other acrylics, even with highly sensitive pigments thinned at a 1:100 ratio.

The Tests

We tested GOLDEN Heavy Body, Fluid, High Flow and OPEN Acrylics on both wood panels and aluminum plates primed with GOLDEN Gesso. In terms of color, we chose Yellow Oxide (YO) and Phthalo Blue Green Shade (PBGS) as typical of our paint lines overall, and Raw Umber (RU) and Anthraquinone Blue as being outliers that were among the most water sensitive. The paints were tested as-is and blended with water in volume ratios of 3:1, 1:1, 1:3, 1:20 and 1:100.

The testing was extremely strenuous and went far beyond what anyone would normally encounter. Adhesion was tested at 24 hrs and 1 week using ASTM’s Cross Hatch Adhesion Test (D3359). Water sensitivity was tested at the same intervals, using a cotton swab dampened with water and rubbed 10 times while pressing downward with very firm pressure, as well as brushing another area 20 times with water. The only exception was OPEN, which we tested at 1 and 2-week intervals instead, due to its slow curing time. Additionally, we tested for any color lift when brushing on Heavy Body or OPEN Titanium White, Polymer Varnish, or our Isolation Coat recipe. Finally, we pre-made 1:5 and 1:10 blends of water with either Fluid Matte Medium or High Flow Medium, then used those to thin Raw Umber and Anthraquinone Blue at 1:20 and 1:100 ratios. These were then tested for any sensitivity at 24 hrs and 1 week. In all, there were some 175 samples generating well more than 1,000 data points.

Some Limitations

While our results should hold true on top of other GOLDEN Paints, Grounds, Gels, and Mediums, because there are so many variables and combinations, it is always important to test for your application. Also, keep in mind that glossy acrylic surfaces might cause some wetting issues when applying highly diluted paints. We provide some suggestions for dealing with that later on. We did not look at direct adhesion to non-absorbent surfaces like plastic and metal, so always test those before use as well. For suggestions on how to test, please see the Just Paint article Will It Stick? Simple Adhesion Testing In Your Studio. Furthermore, our results would not be applicable to materials like glass or glazed ceramics that acrylics inherently have poor to moderate adhesion to without the use of special primers, mediums, or preparations. Finally, we did not test other brands of acrylics and cannot speak to how those would perform in similar tests.

Results

Adhesion

As mentioned, even thinning the paint with 100 parts water did not lead to any adhesion failure. (Image 1) Of course, adding that much water will also turn paint into a stain with no film thickness to speak of. By that point the paint is soaking into the absorbent acrylic gesso the same way watercolor washes will get absorbed by paper. However, all the acrylic blends, even the 1:3 or 1:20 ratios, which still dried with a discernible film, performed well. At least on an acrylic ground, adhesion is simply not a concern.

Sensitivity to Water and Other Products

Colors with a Typical Range of Sensitivity: Phthalo Blue GS and Yellow Oxide

Between these two typical colors from our Heavy Body, Fluid, and High Flow lines, only Phthalo Blue GS showed some slight water sensitivity at the 24 hr mark, and that disappeared when it was retested after 1 week. (Image 2)

Image 2: Test Panel of GOLDEN Heavy Body Pthalo Blue Green Shade and Yellow Oxide thinned 3:1, 1:1, 1:3, and 1:20 paint to water. Adhesion tests were done after 24 hrs, while cotton swab water sensitivity tests were done after 24 hrs. and 1 week.

Our OPEN line, by contrast, was tested at 1 and 2 weeks since it takes longer to lock down. While the cotton swab was able to pick up more color, the color was largely restricted to a concentrated dot, which is more about the relatively soft film being physically rubbed off than color lifting per se. It is also important to keep in mind that the cotton swab test is a very aggressive one, and this degree of rubbing would rarely be encountered. (Image 3)

Image 3: Test Panel of GOLDEN OPEN Pthalo Blue Green Shade and Yellow Oxide thinned 3:1, 1:1, 1:3, and 1:20 paint to water. Adhesion tests were done after 1 week while cotton swab water sensitivity tests were done at1 and 2 weeks.

Paints with Higher Sensitivity: Raw Umber and Anthraquinone Blue

Because we had so few failures with Phthalo Blue and Yellow Oxide, we decided to continue the testing using Anthraquinone Blue and Raw Umber, a pair of highly sensitive colors representing a ‘worst-case scenario’. We left OPEN out of these additional rounds since it presented such unique issues due to its slow drying formula. Instead, we opted for High Flow and Fluid Acrylics, since they are frequently used for wash applications, plus any results from the Fluids would be applicable to Heavy Body as well. Finally, we increased the maximum water additions and tested colors at both 1:20 and 1:100 ratios; far past what almost anyone would use.

Water Sensitivity

In judging water sensitivity, we wanted to add something less physically aggressive than the cotton swab test. We therefore decided to brush water 20 times over a small section on the test panels, then wipe the brush on a clean piece of paper to see if it had picked up any residue. And indeed, for the Raw Umber in particular, you could see a very light wash of color left on the sheet. But even then, the area on the panel where we had brushed, appeared untouched. For most artists under most circumstances, we felt, this amount of sensitivity could be acceptable, especially as most people would be doing far fewer passes than we did. And even at the higher level, it certainly was not anywhere near the warnings of complete lifting expressed by some. In the cotton swab area, enough color was lifted to still be a concern if someone was rubbing or working the surface more forcefully.

Image 4: Test panel showing Fluid and High Flow Raw Umber thinned 1:20 and 1:100 with water and applied as one or 3 coats. All testing was carried out after one week.

Sensitivity to Other Products

Of course, simply rubbing or brushing water by itself on a surface is just one type of testing, and is not something artists might do frequently outside of cleaning the surface, or using very watered down washes. So we also tested other types of applications: brushing on a layer of paint (in this case, Heavy Body and OPEN Titanium White), adding an isolation coat of 2 parts Soft Gel Gloss and 1 part water, brushing on GAC 100 and 500, and finally applying our Polymer Varnish. (Image 5)

Image 5: Test panel of GOLDEN Fluid Anthraquinone Blue and Raw Umber thinned 1:100 with water. All tests conducted after 1 week.

While the Titanium White brush outs all looked fine, the other products lifted color to one degree or another, varying by paint line and pigment. While none entailed a significant loss of color, an artist might certainly notice it while applying an Isolation Coat or brushing a medium over adjacent colors. It is also worth noting that the Polymer Varnish lifted the most color, likely due to its unique composition and amine levels. Much more problematic, however, was the appearance of dark rings around the outer edge where many of the clear mediums and varnish had been applied, in particular, to highly diluted Raw Umber; almost certainly evidence of suspended pigments that were lifted and deposited there in the process of drying. (Images 4 & 5) However, even if limited to Raw Umber, it was clearly an issue that needed to be remedied.

Image 6: Test panel of GOLDEN High Flow Indigo Blue (Anthraquinone) and Raw Umber thinned 1:20 with water. All tests done after 1 week.

One Easy Fix for the Increase in Sensitivity

We entered round 3 of our testing hoping that a simple blend of some acrylic medium and water could be an easy solution to the earlier issues. We therefore tested mixing Fluid Matte Medium or High Flow Medium at both 1:5 and 1:10 ratios with water, then blending each of those at 1:20 and 1:100 ratios with paint. These were brushed onto an acrylic gessoed panel where, after just 24 hrs, we saw almost no evidence of sensitivity outside of a handful of cases, and even those all showed improvement when reexamined a week later. Also, when we tested the cotton swab on top of an area with an isolation coat, even if we rubbed 30 times while using a lot of pressure, we could not get any amount of color to lift. And reassuringly, the unsightly dark rings that we saw earlier were no longer an issue. Lastly, and perhaps just as importantly, these thinned down mixtures of water and medium really felt indistinguishable from using water alone.

Image 7: Test panel showing various blends of GOLDEN High Flow and Fluid Anthraquinone Blue and Raw Umber blended with 1 part GOLDEN Fluid Matte Medium or High Flow Medium mixed with either 5 or 10 parts water. All applications were done after 24 hrs. and water sensitivity tests were done at 24 hrs and 1 week.

Changes to Other Properties

Handling, Sheen, Wetting, and Stability in Storage

When thinning paints with water more than just adhesion or sensitivity can be impacted. The entire feel and performance of the paints change as well, and can often be a major reason for wanting to use or add mediums rather than water alone.

Handling: the more water that is added, the more the paint will feel….well, watery! Which of course is not a surprise, but certainly something to be aware of. Otherwise the rich transparent glaze you were hoping for might appear more like a thin watercolor wash after it dries. And of course, thinned down paint can cause runs on a vertical surface, puddle on a flat one, or cause things like paper to buckle and warp. So always do some testing to make sure that just adding water gives you what you want. If it doesn’t, and you want something thicker in feel or richer in binder, then try experimenting with different ratios of water and acrylic medium until you find the balance you are looking for. Or even try using our High Flow Medium by itself, which is the thinnest of our mediums and can often act as an alternative to water while maintaining a high binder level and imparting a glossy appearance.
Sheen: The more water you add, the more matte the paint will become, regardless of what sheen or line of paint you start with. Chart 1 shows the change in gloss for two of the colors as they were increasingly thinned down. This can also impact the appearance since matte colors appear lighter and less saturated. If this is a concern, adding mediums can help give you more control in this area. However, at the ratios that we blended mediums and water in our tests (1:5 and 1:10), the results still appeared fairly matte, so you would need to continue adjusting the ratio to get the sheen you want. Or again, look to using High Flow Medium by itself for a glossy film.

Chart 1: Graph showing decrease in gloss with greater additions of water.

Wetting: The acrylic lines are formulated with an optimal level of surfactants that, among other things, will help them wet-out a variety of surfaces more easily. As more water is added, these surfactants get diluted and you might find that the paint begins to bead up or crawl on some surfaces, like metal or some plastics, or resists penetrating into raw canvas. To solve this you could try mixing more medium with the water, or add a tiny amount of our Wetting Agent into the water you plan to use, making sure to follow the directions very carefully as this is a highly concentrated material and adding too much will cause other problems.
Storage: The more watered down the paint, the more the pigment particles will start to separate out and gravitate towards the bottom. To prevent this while working, you will need to stir periodically to keep them in suspension. It is also best to only mix up as much as you need in the very short term as very thinned out paints can develop mold since the overall pH starts to move towards neutral and the biocides that were originally added have become too diluted to be effective. If you need to make up larger quantities, always use distilled water and store in clean uncontaminated containers. Also, adding a very small amount of unscented household ammonia (about a capful per gallon of water) will maintain an alkaline pH that discourages spoilage.

Our Recommendations

Given all of the results, where do we end up in terms of recommendations? While there is nothing wrong with our older advice of a 1:1 ratio, which is both easy to remember and eyeball in mixtures, it remains an extremely conservative limit and we want people to feel more confident and comfortable exploring mixtures well above that. We hope the guidelines below will help:

First, just a reminder that the following guidelines are for using GOLDEN Heavy Body, Fluids, and High Flow Acrylics on top of GOLDEN Gesso, Mediums, or Grounds, as well as other layers of those same paint lines. We can not speak to the performance of other brands of acrylics.

And, even more importantly: Always Test for your application to make sure it meets your needs and expectations.