Acrylic mediums explained: retarders, gel mediums, flow improvers and glazing

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Walk into an art shop and the acrylic medium shelf can be bewildering: gels, pastes, fluids, retarders, flow aids, all in gloss, satin and matt. The good news is that most of them are variations on one idea. This guide explains what each type does, so you can buy only what you need.

What a medium actually is

Acrylic paint is pigment held in an acrylic binder. Most mediums are that same binder without the pigment, in different thicknesses and finishes. Mixing them with paint changes its consistency, transparency, sheen or drying time without weakening the paint film the way lots of water can. That point matters: water is fine for thinning a little, but adding a great deal of it spreads the binder thin, which can lead to a weak, powdery film. If you want very thin paint, use a fluid medium for part or all of the thinning.

Retarders and slow-drying mediums

What they do: slow down drying so you can blend for longer.
How to use: add small amounts. Makers' recommendations vary, and some warn that too much can leave a film that stays tacky or soft. One manufacturer suggests around one part retarder to seven parts paint as a typical addition. Follow the label for the product you have.
Alternatives: some ranges sell "open" or slow-drying paints, which stay wet much longer without additives.

Gel mediums

What they do: gels are thick, like paint without the colour. They thicken paint, extend it without losing body and make it more transparent.
Uses: impasto and texture, holding knife marks and brush strokes, making paint go further, and as a glue for collage.
They come in gloss, semi-gloss and matt, and from soft to extra heavy.

Fluid and glazing mediums

Fluid mediums are pourable versions of the binder. They thin paint while keeping the film strong.
Glazing medium is designed to make paint transparent and often slower drying, for building thin layers of colour.
Gloss versions keep colours rich; matt versions reduce shine.

Flow improvers

What they do: they reduce the surface tension of water, so thinned paint flows more evenly and soaks into a surface without beading.
Uses: washes, smooth flat colour, fine line work, and on brushes that drag.
Use small amounts. Flow improver is an additive, not a binder, so it does not strengthen the paint.

Texture and modelling pastes

Thick, often opaque pastes that dry hard and can be built up, carved or sanded once dry (wear a dust mask if you sand).
Good for building texture before painting over it.

Pouring mediums

Designed to let paint flow and level for poured paintings, with less crazing and cracking than paint thinned with water. See the pouring guide in this room.

How to glaze with acrylics

Glazing means painting thin, transparent layers of colour over dry paint, so light passes through and the colours combine optically.

Make sure the layer underneath is dry.
Mix a small amount of paint, ideally a transparent colour, into a glazing or fluid medium.
Brush it on thinly and evenly with a soft brush.
Let it dry before the next layer. Acrylic glazes dry quickly, so you can build several in a session.
Use glazes to warm up or cool down areas, deepen shadows or unify a painting.

Finishes

Gloss mediums make colours look deeper and richer.
Matt mediums reduce glare, but can make dark colours look slightly lighter or cloudy if used heavily.
You can mix gloss and matt to get a satin finish.

Where to start

If you buy only two things, choose a gloss or matt fluid medium for thinning and glazing, and a regular gel medium for texture. Add a retarder if drying speed is your biggest frustration. Which medium has changed the way you paint, or confused you most? Tell us in the comments.