Airbrush thinner and flow improver solve two different problems and painters often reach for the wrong one. Thinner reduces paint viscosity so it atomizes through a narrow nozzle instead of clogging it. Flow improver reduces the paint's surface tension so it lays down smoothly instead of beading or spattering, even when the viscosity is already correct. A paint that sprays in dots instead of a fine mist usually needs flow improver, not more thinner, and adding thinner to that problem just makes the dots smaller and thinner them without fixing the beading.
What thinner actually does
Straight from the bottle, most acrylic paints are too thick to pass through an airbrush's small nozzle without clogging, even a general-purpose 0.35mm tip. Airbrush thinner(affiliate link) is a water-based medium formulated to reduce that viscosity without breaking down the paint's binder the way plain water can over time. A rough starting ratio is one part paint to one part thinner, adjusted by feel: paint that still clogs a fine nozzle needs more thinner, paint so thin it barely covers in one pass needs less.
Every brand's paint is formulated slightly differently, so a thinner built for one line is not guaranteed to behave the same with another. Using the same brand's own thinner, where available, avoids the small risk of pigment separating out or the paint curdling in the cup, which does occasionally happen mixing across brands.
What flow improver fixes
Flow improver, sometimes labeled a flow aid or a wetting agent, lowers the paint's surface tension so it spreads into a continuous film instead of pulling back into small beads on the model's surface. This shows up as a stippled or dotted texture even when the paint is thin enough to spray cleanly, which is the tell that the problem is surface tension, not viscosity, and more thinner will not solve it.
A drop or two of flow improver per mixed batch is usually enough. Overdoing it can make the paint spread further than intended and lose crisp edges around masks, so it is worth testing on a spare model or a scrap sprue before committing to a full model.
Cleaning solution is a separate bottle
Thinner and flow improver both go into the paint cup before spraying. Cleaning solution goes through the airbrush between colors and at the end of a session, and using thinner as a substitute for proper cleaner tends to leave residue that builds up in the nozzle over time. An acrylic airbrush cleaner(affiliate link) flushed through between colors, backed up by a nozzle cleaning brush kit(affiliate link) for the inevitable stubborn clog, keeps a fine-nozzle airbrush spraying consistently far longer than thinner alone.
Getting the ratio right for your airbrush
A wider nozzle, in the range used for primer and base coats, tolerates thicker paint and needs less thinning than a fine detail nozzle used for highlights and glazes. If you own one airbrush and switch tasks often, keep notes on the ratio that worked for each job rather than re-guessing every session; consistency in mixing is what separates a smooth base coat from a blotchy one. Our airbrush kit roundup and the Iwata Eclipse vs Neo comparison go into which nozzle sizes suit which jobs if you are still building out a kit, and the Vallejo brand hub covers the Model Air and Game Air style thinner-ready lines that most airbrush painters start with.
FAQ
Do I need both thinner and flow improver?
Not always. Thinner is close to mandatory for most acrylics through an airbrush. Flow improver is a fix for a specific symptom, spattering or beading, and is not needed if the paint is already laying down smoothly.
Can I use water instead of airbrush thinner?
Water thins paint in a pinch but lacks the binder-friendly medium in a proper thinner, and repeated use of plain water can weaken the paint's adhesion over many thinned coats.
Why does my airbrush still clog after thinning the paint?
Clogging after thinning usually points to a dried nozzle tip, a paint that has partially dried in the cup, or too little thinner for that specific nozzle size, rather than a flow improver problem.
How much flow improver should I add?
A drop or two per mixed batch is a reasonable starting point. Test on scrap first, since too much can spread the paint beyond a crisp masked edge.