What it does: Holds pigment particles together and bonds paint to the surface
The binder is the chemical component of paint that suspends the pigment and, once dry, forms the solid film that adheres to your surface. In acrylic paints, the binder is acrylic polymer emulsion — microscopic plastic particles suspended in water. When the water evaporates, the polymer particles fuse together into a flexible, durable film.
The binder determines many of the paint’s handling properties: how it flows, how quickly it dries, how flexible or brittle the dried film is, and how well it adheres to different surfaces.
Understanding binders matters for airbrushing because adding too much water can break down the binder ratio, reducing adhesion. This is why flow improver is preferable to plain water for thinning — it maintains the binder concentration while reducing viscosity.
Related: Pigment · Viscosity · Flow Improver · Reducer