What Is Acrylic Medium, and What Does It Actually Do?

Acrylic medium is a formulated additive mixed into acrylic paint to change specific working properties — thickness, sheen, drying time, transparency, or texture — without simply diluting the pigment concentration the way adding plain water does.

Why not just add water?

Water thins acrylic paint, but it also dilutes the binder along with the pigment, which can weaken the paint film's durability and adhesion if overdone. Acrylic medium is built from the same binder resin the paint itself uses, so it extends or modifies the paint without breaking down that binder structure the way excess water can.

Common types of acrylic medium

• Gloss medium — increases sheen and can improve color vibrancy and transparency.
• Matte medium — reduces sheen for a flat finish, useful when a glossy look isn't wanted.
• Gel medium — thickens the paint significantly, useful for heavy texture or impasto techniques, while keeping it workable.
• Retarder — slows drying time, useful for acrylic's normally fast dry time when you need longer working/blending time (see also our guide on slowing acrylic drying time).
• Flow improver — reduces the paint's surface tension for smoother, more even application, especially useful for fine detail work.

How to use it

Medium is mixed directly into the paint before application, in a ratio that depends on the specific product and desired effect — start with a small amount and adjust, since mediums vary significantly in strength between brands and types.

Medium vs. varnish — a common confusion

Medium is mixed INTO paint before or during application to change its working properties. Varnish is applied AFTER the paint is fully dry, as a separate protective topcoat — they're not interchangeable, and using one where the other is called for won't achieve the intended result.

Who actually needs this

Casual acrylic painters doing straightforward projects often don't need medium at all — it becomes genuinely useful once you want a specific effect (heavy texture, extended working time, glazing transparency) that the paint alone, even thinned with water, can't achieve.