One of the most critical problems in the manufacture and use of acrylic paint is the development of foam. It can cause structural problems in the dry paint film by weakening it and can create a more permeable paint surface capable of imbibing dirt and other discoloring pollutants. But the primary problem with foam is that it can cause a clouding or fogging in an otherwise transparent acrylic surface.
Understanding how foam is created leads to an understanding of how to deal with and potentially avoid it. While it is easy to suggest not to create foam in the first place, this is sometimes difficult to achieve with waterborne acrylics. No matter how careful one is with their working technique, the simple movement of a paint filled brush rising with high points on the work and slapping down into valleys is enough to create a dried cloud of foam. This is usually only a problem when working in transparent layers or applying a transparent coat of varnish.
The Source of Foam
Foam generation in waterborne acrylic paints and mediums comes from several sources. Yet, all of these have, as their primary basis, the use of chemical wetting agents called surfactants. From the same family of chemicals as detergents, surfactants tend to be extraordinarily good at creating foam. However, they are indispensable. By reducing the surface tension of water, they serve to stabilize the acrylic polymer, and are also required for dispersing pigments into the acrylic.
Optimizing surfactant use is the most complex and critical part of paint formulation. Their high propensity for foam generation requires tight controls. Each pigment, each system, each material requires different levels and types or combinations of surfactants to make a stable paint. While it is sometimes possible to substitute a lower foaming product, it is important not to sacrifice overall paint performance.
Creating foam is usually an unavoidable consequence of the high speed mixing and shearing processes required to disperse pigments and make paints. Chemical defoamers, which act to both reduce foam generation as well as break foam structure once it occurs, are part of every waterborne paint formulation. Thicker paints allow for the incorporation of stronger defoamers, yet the thicker the paint, the more resistant the foam is to dissipating. It may take up to several weeks or months for foam that is stirred in to rise out of heavy bodied acrylics. Thinner bodied paints, like GOLDEN Fluid Acrylics, require a much more delicate defoamer to avoid other film defects that too much or too strong a defoamer can cause. Shaking or strong agitation of these colors during mixing may cause a froth of foam like the head of a beer.
Foaming in Varnish
It has been the practice of many artists and paint companies to recommend using polymer medium as a varnish for acrylic paintings. We have seen many cases where this has caused irreparable foam damage to the surface of the painting. It is nearly impossible to apply a layer of polymer medium without creating foam, which will not dissipate and is not removable. For this reason we recommend using GOLDEN Soft Gel (Gloss) if an isolating coat is needed or when strong defoaming is required. It contains stronger defoamers, which even allow for a degree of scumbling, while drying almost completely foam free. The Soft Gel should be thinned with about 20% to 40% water for this purpose.
GOLDEN Varnishes are formulated to avoid the foaming problems typical of those based on acrylic emulsions. GOLDEN MSA Varnish and Polymer Varnish are solution and colloidal polymers respectively. Both are made with strong defoaming capability. However if foam does occur, these varnishes are removable without damaging the acrylic surface. (See Varnish Review, JUST PAINT #3)
Mixing Can Generate Foam
When mixing varnish or colors, it is important to follow techniques that reduce foam generation. If using power equipment, avoid creating a vortex, a tornado like funnel that occurs during high speed mixing. Always mix from the side of the vessel and not in the center. When hand mixing, avoid stirring in colors with a whipping action. It works best to mix colors by spreading them together with a palette knife on a palette. This allows more surface area to contact the air and for the action of the defoamers to take over. Avoid shaking the product. Although this is a quick way to mix, a gentle stir will certainly cause less foam. If possible, mix colors ahead of time. This allows time for foam bubbles to rise and be burst by the de-foamer, especially in heavier bodied colors.
Application Affects Foam
Application tools and technique can also influence the amount of foam generated. Generally, more vigorous application increases the tendency for foam. Speed of brush across canvas, force applied and the relative stiffness of the brush all contribute. When applying varnish, use a clean soft brush. Although many artists swear by a “foam” brush, our tests show that its name suits it well, as it does seem to cause excessive foam on the surface. Rollers also tend to produce a great deal of foam upon application. On heavily textured work, applying a foam-free paint layer or varnish may be impossible without the use of a sprayer. In very thin applications of paint or varnish that are still wet, foam may be removed by pricking the bubbles or blowing across the surface.
Testing new materials, brushes, surfaces and techniques prior to actual use will help. This will allow the artist to anticipate foam problems and develop methods to circumvent them.
If you have followed all the precautions and still have problems with foam generation, we have several defoamer types that may suit your application. So, if foam is bubbling through your work give us a call, maybe we can help, if not, maybe we can console.