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Airbrush and Oil Mediums: Thinning Without Ruining the Paint
Two categories at the technical end of the counter, and both get thinned with the wrong product regularly. Airbrush work fails by clogging and spattering; oil work fails months later by cracking. Both are avoidable with the right medium.
Airbrush medium
An airbrush nozzle is a fraction of a millimetre across, and acrylic straight from the bottle is far too heavy to pass through it. Something has to thin it.
Water alone dilutes the binder, which produces the same weak, chalky film it produces anywhere else — plus, at airbrush dilutions, paint that will not stick at all. Airbrush medium thins while adding binder, and it slows drying slightly so the paint does not cure in the nozzle between passes.
Our oil and airbrush mediums range lists what each product does and the bottle volume.
Consistency
Milk is the standard reference, and it is a good one. Thicker than milk spatters and clogs; much thinner runs on the surface and loses coverage.
Test on scrap at working pressure before going near the piece. Different colours need different dilution — heavily pigmented ones need more thinning than transparent ones.
Flow improver is not thinner
A common confusion. Flow improver reduces the surface tension of the paint so it levels and spreads; it does not reduce body. Thinner reduces body without addressing surface tension.
For airbrushing, you generally want both, and airbrush-specific mediums usually combine them. Adding flow improver alone to thick paint will not get it through the nozzle.
Cleaning
Flush between colours and strip the airbrush at the end of every session. Acrylic that dries inside a nozzle is difficult to remove and is the main cause of airbrushes that “stopped working”.
Keep a dedicated cleaner rather than using water alone, and never leave paint sitting in the cup.
Oil painting mediums
Oil mediums do a different job: they control drying time, gloss and the handling of the paint.
Alkyd-based mediums
Speed drying markedly — a passage that would take days can be touch-dry overnight. They also increase gloss and improve flow. Useful for glazing and for anyone who cannot leave a painting for a week between layers.
Oils alone
Linseed, walnut, poppy. These slow drying and increase gloss and flexibility. Different oils yellow to different degrees, which matters in pale passages.
Solvents
Reduce body and speed drying by evaporation, and they reduce the binder proportion. They belong in the early lean layers only.
Fat over lean
The rule that prevents cracking, and the one most often broken.
Each successive layer must contain more oil — be “fatter” — than the layer beneath. Fatter layers stay flexible longer. A lean, fast-drying layer over a fat, slow-drying one dries and shrinks while the layer below is still moving, and it cracks. That cracking may not appear for months, which is why the mistake is rarely connected to its cause.
In practice: start with paint thinned by solvent, and increase the proportion of oil or alkyd medium in each subsequent layer.
Drying times, honestly
Oil paint is touch-dry in days and fully cured in six months to a year depending on thickness and pigment. Varnishing before then traps solvent in the film.
If a piece has to be varnished early, a retouch varnish is the interim option, with a final varnish later. It is worth being clear with anyone commissioning work that this is how oil paint behaves.
Safety
Solvent-based products need ventilation — an open window, and not a sealed room. Wear a respirator when airbrushing anything for any length of time; the particle size is small enough to reach deep into the lungs.
Oily rags: rags soaked in linseed oil can self-heat and ignite. Lay them flat to dry outdoors, or store them in a sealed metal container with water. This is a real hazard and not a theoretical one.
Where to start
For airbrushing acrylics, one bottle of airbrush medium and a dedicated cleaner. For oils, one alkyd medium and one solvent, which covers lean underpainting through to fatter finishing layers.
Everything is listed with what it does under oil and airbrush mediums, with the acrylic side under pouring and fluid mediums. We are on 3rd Avenue in Brooklyn — ask us at the counter, see the FAQ, or browse the range.
From our shelves
Products in this store that relate to what is covered above:
Prices are in US dollars. Delivery timings are on our shipping policy page and the return and refund policy sets out the window if something is not right.
Key points
- Airbrush medium thins and improves flow without breaking the binder or clogging the nozzle.
- Aim for a milk-like consistency; thicker spatters, thinner runs and loses coverage.
- Flow improver and thinner are different products — one reduces surface tension, the other reduces body.
- Oil painting mediums change drying time and gloss; alkyd-based ones accelerate, oils alone slow it down.
- Follow fat over lean in oils: each layer must contain more medium than the one below it.
- Ventilate properly for solvent-based products, and dispose of oily rags safely — they can self-heat.
