How to Thin Acrylic Paint Without Ruining It

Somewhere along the way, beginner painters picked up a hard rule: never add more than 30% water to your acrylics, or the paint will flake right off the canvas. It’s tidy, it’s memorable, and according to the people who actually manufacture acrylic paint, it’s not true.

GOLDEN’s paint chemists put this folk wisdom to the test in their own lab and reported the results publicly: their old guidance was a deliberately conservative 1:1 ratio of paint to water, and in testing they saw no adhesion failure “no matter how thinned down with water” when the paint went onto an acrylic-gessoed surface — even at one part paint to 100 parts water. Liquitex, meanwhile, doesn’t publish any maximum water percentage at all; the company’s official line is simpler: use acrylic mediums when you thin, because mediums maintain the paint’s integrity, color strength, and archival quality.

So water isn’t the enemy. Water alone, pushed too far for the wrong job, is where the trouble starts. This guide shows you where that line is — organized by what you’re actually trying to make, because the right thinning method depends entirely on the use case.

When Water Alone Is Totally Fine

For everyday brushwork — loosening a dollop of heavy-body paint so it flows off the brush — water is the right tool. It’s free, it doesn’t change the paint’s handling much in small amounts, and GOLDEN’s own testing called a 1:1 ratio “very safe” for their paints on gessoed surfaces.

Here’s the mental model that makes everything else click: water dilutes everything in the paint — pigment, binder, and additives alike. An acrylic medium thins the paint while adding binder back in. So think of water as the thinning tool for small jobs, and medium as the thinning tool for big jobs. Mixing your colors first and then thinning the mixture keeps the color consistent across the batch.

What Actually Goes Wrong When You Push Water Too Far

Over-thinning with water doesn’t announce itself while you’re painting. The problems show up later. Here’s what the manufacturers say happens:

The paint film gets weaker

Liquitex puts it plainly in its mural-painting guidance: “too much water will weaken the paint film,” and over-thinning gesso with water alone “will cause poor adhesion.” The binder that glues everything to the surface gets spread too thin to do its job properly.

Colors can lift later

GOLDEN’s lab tests found that highly diluted paint becomes more water-sensitive — especially colors like Raw Umber, which has a naturally high clay content, and dye-like pigments such as Anthraquinone Blue. The practical risk: brushing varnish or another medium over a fragile, water-sensitive layer “can cause some color lift.”

Everything goes matte and washed out

Two predictable shifts come with heavy dilution. GOLDEN: “The more water you add, the more matte the paint will become, regardless of what sheen or line of paint you start with.” Liquitex: “Diluting reduces pigment color strength” and “increases transparency.” So that glossy, saturated color in the wet puddle will dry flatter and lighter than you expect — plan for it rather than being surprised by it.

Thin paint beads up on slick surfaces

Water-diluted acrylic “might bead up on glossier surfaces,” GOLDEN notes, because of surface tension. A touch of medium in the water usually fixes the beading. Conversely, on an absorbent surface like acrylic gesso, highly diluted paint behaves more like watercolor: the surface helps “hold highly diluted paint to the surface,” which is exactly why washes feel so natural on paper.

Thin by Use Case: Washes, Glazes, Pours, and Airbrush

So many thinning guides contradict each other because they answer “how much water?” without asking “for what?” A wash and a pour are different materials problems. Here’s what each one wants:

Washes: go water-heavy

A wash is made by thinning paint with a lot of water — GOLDEN’s words. In their published tests of washes and glazes, washes ran around one part paint to 90 parts water or medium, and the company states flatly that “there are no rules for ratios with either water for thinning, or medium for extending.” Practical tips: fluid or high-flow acrylics make washes far easier than heavy-body tubes do, and if you’re nervous about a weak film, stir in a little thin medium (High Flow Medium or Fluid Matte Medium) — or brush a thin layer of medium over the dry wash to lock it in. For smooth blended transitions, a thinned wash underlayer gives you working time on canvas.

Glazes: reach for medium, not water

This is the big switch. GOLDEN defines a glaze as a “binder rich mixture typically having a very small amount of paint blended with a much larger amount of medium” — made by extending paint with translucent medium, not by thinning it with water. As a starting point, their Acrylic Glazing Liquid blends about 1:1 with paint (which can still look surprisingly opaque, so add more medium until it’s right), and there’s no fixed ratio to obey. Liquitex gives the same structural advice: build glazed effects from transparent colors — their Acrylic Ink or Soft Body lines — mixed with Professional Glazing Medium.

Pouring: use a pouring medium

Pouring is the one technique where plain water genuinely gets you into trouble, because you need a huge volume of fluid paint that still dries into a strong film. GOLDEN’s recommended route for heavy-body paint: pre-mix one part thin medium (like GAC 100) with 1.5 parts water, then use that mixture to thin the paint as much as you like — you get pourable paint instead of a weak color stain. For their GAC 800 pouring medium, the starting ratio is 10 parts GAC 800 to 1 part Fluid Acrylic, and pours should stay thin: in GOLDEN’s pouring techniques guide they warn that in dry climates, thick pours develop “crazes” — fissures where the skin shrinks and tears over still-wet paint underneath. Liquitex’s recipe: at least a tablespoon of Soft Body color per cup of Pouring Medium, blended gently with a palette knife and left to sit 10 minutes so bubbles escape — and one firm rule: “do not mix water with the Liquitex Pouring Medium.” Give pours at least a full day to dry, two for thick films.

Airbrush and spray: use airbrush medium

For spray work, Liquitex says thin to “a light cream consistency or thinner” — and in its airbrushing tips guide recommends its Airbrush Medium over water for soft-body paints, because it keeps the colors’ “flexibility, durability and adhesion” while adding flow and slowing drying so the nozzle doesn’t clog. Its Acrylic Inks are fluid enough to spray straight from the bottle. One more practical step: strain thinned soft-body paint before it goes in the cup. (A diluted Flow Aid additive is a second option — up to 80%, about 1:1 with Soft Body — but remember it weakens the film, so mediums stay the safer default.)

7 Mistakes Beginners Make When Thinning Acrylics

1. Trusting the 30% rule instead of the use case

GOLDEN’s chemists specifically called out the “magical mark at 30%, others at 50” as rules “presented without citing or showing any evidence.” The real dividing line the company gives is 1:20 — beyond that ratio, they recommend adding at least 1 part medium per 10 parts water to keep the film strong. Structure your thinning around washes vs. glazes vs. pours, and the numbers sort themselves out.

2. Using retarder as a thinning medium

Retarder slows drying; it is not a thinner. GOLDEN warns not to exceed 15% retarder in the mixture, “as it will result in a surface that will not lose its tack.” If you want both extended working time and thinner paint, their Acrylic Glazing Liquid does both jobs with “no restrictions on amounts.”

3. Shaking instead of stirring

Vigorous shaking whips air into thinned paint and additives, and foam in the mixture means foam in the film. Liquitex’s instruction for diluting Flow Aid applies generally: “stir slowly to avoid foaming. If it does foam, leave it to sit while the foam disappears.”

4. Judging the color from the wet puddle

Wet thinned paint lies. Dilution increases transparency and reduces color strength, and more water pushes everything toward matte. Test the thinned mixture on scrap, let it dry, and judge the dry swatch — it’s the only honest preview.

5. Pouring it on thick

Thick pours look luxurious and dry badly. GOLDEN’s crazing warning is specific: liquid acrylic skins over during initial drying while the paint underneath is still wet, “the skin shrinks and tears apart” — and dry climates make it worse. Keep pours thin and even; give them a full day (two for thick films).

6. Storing pre-mixed thinned paint like it’s paint from the tube

Heavily diluted paint is mostly water, and water can grow things. GOLDEN’s advice for larger batches: “always use distilled water and store in clean uncontaminated containers” — as pH drifts toward neutral and the biocides dilute out, mold becomes a real risk. Liquitex specifies distilled water when diluting its Flow Aid concentrate, too. For day-to-day brushwork, tap water is fine; the distilled rule is for mixing and storing.

7. Forgetting the surface does half the work

The same thin mixture behaves like a stain on absorbent gesso and beads up on a slick, glossy ground. If your wash slides around or won’t grab, the surface — not your ratio — is usually the culprit. A coat of matte medium over a dry wash also buys durability for pieces you plan to varnish.

Your Thinning Workflow, Step by Step

Put it together and it takes about two minutes:

Step 1: Name the job

Wash, slight loosening, glaze, pour, or spray? The job decides the thinner — water for the first two, medium from glaze onward.

Step 2: Thin gradually and test dry

Add water or medium a little at a time, brush it on scrap, and let it dry fully. The dry swatch is the truth; the wet puddle is advertising.

Step 3: Cross the 1:20 line with medium, not water

Once you’re adding more than about twenty parts liquid to one part paint, GOLDEN’s tested guidance applies: at least 1 part medium per 10 parts water keeps the film strong. For pours and glazes, medium should have been your choice from the start.

Step 4: Stir gently, clean up normally

Stir — never shake — thinned mixtures, and remember Liquitex’s parting advice: water is “the solvent for the acrylic/water emulsion,” so save it for clean up and let mediums do the heavy thinning. Speaking of cleanup, cleaning your brushes properly matters even more when you’re working with thin, watery mixtures that wick deep into the ferrule.