Additives are intended to change the behaviour of acrylic colours. They’re not made of polymer resins and can’t be used alone or as adhesives. They need to be used even more carefully than mediums; be sure to follow the recommended ratio for each product.
Retardant can be mixed into your paint or painted directly onto your work before it dries to slow the drying process. Whether you prefer it in gel form (tubes) or in liquid form (pots), it should be used sparingly, up to a maximum ratio of 10 to 25% of the paint’s volume depending on the brand. Too much retardant will hinder proper drying, and the colour will shrink when the water evaporates from the mix. Colours that are more viscous and colours with gel mediums mixed in take more time to dry. Retardant is not a miracle solution, but it can give you up to 50% more time to work with your acrylics. You can also slow the drying of an acrylic paint by misting it with a bit of water while you’re working. Just make sure you’re not painting on a surface that’s too absorbent.
Spreading mediums can be added to your paints to improve their fluidity. They make it easier to spread the pigments by reducing the surface tension of the paint layer’s surface. This is particularly helpful for watercolour and airbrushing techniques. These mediums should not be used directly to thin colours or mediums; the concentrated medium should be diluted in water (no more than 10% medium).
Thickening gels can help you to achieve effects similar to oil painting by giving your paint lots of body. Add the product bit by bit to your colour until you get the desired consistency. Too much gel can cause the colour to shrink when it dries; it’s best not to go above 50% of the initial paint volume. Marbling mediums can be mixed with medium-viscosity paints to create marbled effects. The colour is diluted directly with the medium until the mix is runny enough to drip off a brush. A mix of 50% paint and 50% medium is usually best.
The marbling technique involves drizzling diluted paint off a brush into water and swirling it with a stick, then transferring the paint onto paper or cloth by laying it gently on the water’s surface.