What a Medium Does That Water Cannot

Thinning acrylic with water and thinning it with a medium look similar in the pot and behave differently on the surface.

Water dilutes the binder

Acrylic paint is pigment held in an acrylic binder. Adding a lot of water thins the binder as well as the paint, and past a point the film that dries is weaker and can lift or chalk.

A medium keeps the binder intact

Mediums add more binder rather than less, so you can thin, extend or change the finish without weakening the paint film.

Matte, gloss and satin

The same medium family usually comes in different sheens, and the sheen carries through to the dried surface. Mixing sheens across a piece is a decision worth making deliberately.

Test first

Mediums change drying time and colour depth slightly. A test on scrap tells you more than a description ever will.