How to thin acrylic for an airbrush: match the paint

How to thin acrylic for an airbrush is the next decision. Match the thinner to that paint, then match the mix to the nozzle, then spray a piece of card. A splash of water will not serve a Tamiya jar, a Vallejo bottle, Humbrol acrylic and a craft pot from a kit in the same way.

Figures here come from public UK guides from Liquitex, modelcalcs.uk, Airfix support, Stardustcolors and GraphicAir, plus the amazon.co.uk listings in our airbrush set comparison. We did not run a lab viscosity test. A qualifying purchase through our links can earn a commission at no extra cost to you.

In short

Thin acrylic for an airbrush by the paint in the bottle, then by the nozzle, then by a test on card. Liquitex ink is used as it comes; soft body goes towards a light cream with airbrush medium and should be strained. Humbrol acrylic usually starts at one to two parts paint to one part of the matching thinner. Milk is a picture of flow, and that picture changes when you swap a 0.5 mm head for a 0.2 mm head.

Thin for the paint you actually have

Acrylic on a UK shelf covers artist soft body, acrylic ink, water-based model colour and the craft acrylics packed with a compressor. They do not share a binder or a pigment load. The thinner has to suit the tin you opened.

The MEEDEN MD-AC300H on amazon.co.uk, ASIN B09K3NGGHT, was £74.95 in our comparison, with twelve 30 ml acrylics and a listed 0.5 mm needle on the AR35. The listing ties those paints to paper, canvas, wood and similar craft surfaces, and it cites ASTM D-4236. That line is an art-materials label: it does not state a viscosity, and it does not approve the paint for food.

Why milk is not a measurement

You cannot set milk consistency on a gauge. modelcalcs.uk describes skimmed milk, then says the ratio still moves with the age of the paint, the colour, the temperature and the humidity. Stardustcolors uses liquid cream for a 0.3–0.5 mm nozzle and milk for a 0.15–0.20 mm nozzle. Neither is a viscometer reading in millipascal-seconds.

Starting points by paint family

Every ratio below is a starting point from the source that published it. modelcalcs.uk labels its table as a guide, and Airfix still tells you to test. Add thinner in small steps.

Water-based model acrylic

For Vallejo Model Color, modelcalcs.uk starts at one part paint to one part Vallejo thinner or distilled water. Vallejo Model Air is used neat, or with about 10–20% thinner. Citadel sits at about one to one with Lahmian Medium or water. Distilled water is for these colours, not for a paint whose maker names a solvent. If neat Model Air already lays down through the 0.3 mm head you fitted, stop. Extra thinner makes a wash.

Tamiya acrylic

modelcalcs.uk starts Tamiya acrylic at one to one, and goes as far as one part paint to two parts thinner, using Tamiya X-20A or isopropyl alcohol. Keep that thinner with Tamiya. Keep Vallejo thinner and distilled water with the water-based colours. Isopropyl alcohol is flammable, so use a small amount in a ventilated room and keep it off a naked flame.

Humbrol acrylic

Airfix support puts Humbrol at one to two parts paint to one part thinner. Acrylic Humbrol takes acrylic thinner or water. Enamel Humbrol takes enamel thinner. The brand name on both tins does not make those thinners interchangeable. If the paint runs, you have gone too thin or the gun is too close.

Artist soft body and kit acrylic

Liquitex acrylic ink needs no thinning. Soft-body acrylic is taken to a light cream with Liquitex Airbrush Medium or diluted Flow Aid, then strained. That advice is for Liquitex products, and it does not transfer to a Tamiya jar. Medium changes the body, and flow improver changes wetting. Thin and strain craft acrylic from a boxed kit, and expect a coarser result through a 0.5 mm needle. A 0.2 mm head is a poor bet for that pigment load. For plastic kits, start from a model paint already aimed at an airbrush, on the airbrush set for models page.

Nozzle size changes the mix

The same pot can pass a 0.5 mm head and stop in a 0.2 mm head. The Timbertech ABPST05, our top pick in the airbrush set comparison, was £69.99 with 4.5 from 6,320 amazon.co.uk reviews. The bullets name a 0.30 mm nozzle mounted, plus 0.20 mm and 0.50 mm heads, a gauge, a water trap, autostart at 3 bar and autostop at 4 bar. Prove the mix on the head you will use for that coat. If you only own the 0.5 mm needle listed for the MEEDEN AR35, do not invent a fine-line ratio for a head that was not in the box.

A modeller's rule of thumb is about 18 psi for acrylics and about 25 psi for enamels. It is not printed on these kits. The MEEDEN spec prints 21 psi. The Timbertech spec prints 4 bar, while one review describes about 24 psi and no storage tank. If the spray spatters, thin the paint before you raise the pressure. The ABPST05 spec says 220 V and does not say BS 1363. One review reports a Euro two-pin plug. We did not measure these guns.

Water, thinner and the bottle

Use distilled water only where the guide for that paint allows a water-based acrylic. Tap water is a weak stand-in for a named thinner. If the tin is silent, test a few drops on card and look at the dried film. A grainy film, or a colour that separates, is the moment to buy the maker's thinner.

Mix in a bottle when you need the same colour twice, and write the ratio, the paint name and the date on it. A cup is enough for one session and one nozzle. If you step away, empty the cup or flush the gun. GraphicAir's note is direct: thick paint clogs, and acrylic should not sit in the gun.

Test spray, then stop

A ratio from a table is a hypothesis. The card is the result. Use the same nozzle, distance and trigger you will use on the work.

1. Fit the nozzle, needle and cap as one set for the size you are mixing for.
2. Stir the paint, then add the matching thinner at the starting ratio for that family.
3. Strain soft body and primer before they go into the cup.
4. Read the gauge with the trigger held down, as the Airbrush Company describes.
5. Spray card or spare plastic at the working distance Airfix describes for Humbrol.
6. If the pattern spatters, add a little more of the same thinner before you raise the pressure.
7. If the paint runs, add paint back or move the gun away, then test again.
8. When the card looks even, stop thinning and write the ratio on the bottle.
9. Flush the gun when you stop, including a short break.

Handling around that test sits on the how to use an airbrush set page. If the gun still spits after a mix that passed the card, more thinner will not find a bent needle or dried paint behind the cap. Empty the cup, clean the head, and only then mix again.

What not to thin into a cake or a wall

The MEEDEN listing names cake decoration as a use of the kit. The twelve 30 ml acrylics are not described as food. Thinning them with water, alcohol or airbrush medium does not create a food colour. Do not spray them onto icing, biscuits, or a board that will touch food.

A hobby airbrush is also the wrong tool for a wall. The MEEDEN capacity field is 7 cm³, and the Timbertech spec prints a tank volume of 7 ml. For a room, look at an airless paint sprayer or a paint roller. Thinning emulsion until it looks like milk, then hoping a 0.3 mm needle will cover a ceiling, clogs the gun.

Hand the job to a professional when the coating is a two-pack or isocyanate paint, or when the work is stripping old paint that might contain lead. A thinned acrylic in a hobby gun is not a booth, and it does not remove lead paint. This article does not set a pressure or a mask for those coatings.