The colour pigments that act as the colourant in acrylic paints are less dense than water, and trying to mix pigments with water usually doesn't work; the pigments stay on top of the water in clumps. To begin the process of making acrylic paint, an acrylic dispersant is added to the mixture and the whole thing is blended together. A homogeneous consistency means the colour is uniform and more vivid. Once the mixture has been blended at the optimum level and for the optimum time, the additives we describe below are added to the paint:
Initiators that begin the polymerisation process (bringing all the acrylic particles in the paint together at room temperature to form a continuous film — in other words, the drying process).
Buffers such as ammonia, which help maintain the pH balance between 8 and 10 that's necessary for the paint's stability (and therefore its long-term durability).
Surfactants and protective colloids, which provide long-term particle stabilisation and particle formation (these assist the drying process).
Defoamers, which balance the foaming properties of the surfactants.
Preservatives that protect against the growth of microorganisms (necessary for the paint's long-term durability).
Thickeners and rheology modifiers are added to achieve the desired thickness and flow properties.
Stabilisers to stop the water-based paint freezing; if the paint freezes, its dispersion breaks down and the paint's structure is damaged.
How acrylic paints dry
Acrylic emulsions are made from all the components listed above, blended with pigment and water. During the drying process the water evaporates, and as it does the spherical polymer particles move closer together. Eventually the particles come together to form a honeycomb structure. Coalescing-solvent additives keep the polymer particles soft during and after the drying process. That allows the particles to pack together as tightly as possible even after all the water has evaporated. When all the water has evaporated and the film has become solid and dry, the paint is said to be "continuous" (without gaps) to the naked eye, though under a microscope it's very likely still to have some porosity. Porosity increases when the paint dries in very cold conditions, because the slower drying rate and the energy deficit prevent the particles being drawn close enough together.
Another cause of a higher degree of porosity is foaming in the paint. The pores often trap preservative cleaning agents, which can cause long-term damage to the paint over time.
A layer of varnish can help prevent these problems arising.
Adding mediums
As you experiment with your new colours, being aware of the mediums available will give you an advantage as an artist. A medium is a substance added to paint to thicken or thin it, change its drying rate, add texture — in short, to change the paint's properties. If your paint isn't behaving the way you want, there'll be a medium to help you.
Changing the body is normally done with gels, pastes, flow improvers or fluid mediums. You can use water if you need to thin your paint a little, but using too much disrupts the paint's structure so much that it can become brittle when dry. Significant changes in consistency are best achieved with flow improvers or fluid mediums, which preserve the paint's ability to form a sound film as it dries. Gels and pastes increase the body of the paint in slightly different ways: pastes tend to add volume and are usually opaque, while gels are viscous and transparent. Regular gel has the same consistency as heavy-body acrylic paint and extends the colour without thinning the body, while heavy gel and extra heavy gel add volume.
Changing the drying time is a useful trick to know about, especially if you're just starting out. Adding a little retarder to your paint will give you more time to work with it and increase what's known as the "open" time.
Varnishing acrylic paintings
Many painters see varnishing as part of the painting process rather than just a "finishing touch". Varnishes are available in gloss, matte and satin (satin can also be made by mixing gloss and matte varnishes).
Which varnish can I use?
Varnish will protect your painting and unify the sheen. All fine art varnishes are suitable for varnishing acrylic work. Acrylic varnishes are made from polymer resins dissolved in an acrylic emulsion and are the fastest-drying varnish of their kind. Solvent-based varnishes dry slowly and are made from natural resins dissolved in solvent. The matte versions of solvent varnishes contain an additional matting agent.
How should I varnish my acrylic painting?
Your painting needs to be clean and completely dry before varnishing . An isolation coat made of two parts soft gel and one part water will let you remove and reapply the varnish layers later without damaging the paint. Keep the work in a dust-free environment. You may want to wipe the surface with a clean damp cloth to remove any dust or dirt on your work and let it dry immediately before varnishing. Use a clean, soft varnish brush and apply your varnish in thin, even layers. It's worth adopting a technique to make sure you're not going back over areas you've already varnished (raking light can help you see where it's glossy and where it isn't).
Once the varnish reaches the "tacky" stage of drying, you can tilt your painting against a wall, painted side inwards, to stop dust settling on the wet varnish without the varnish running (as long as it's been applied thinly). Once the layer is dry you can apply another. As always, several thin, even layers always give better results than a single thick one.
