Painting Exterior Murals

Surface Considerations
Surface Preparation
Painting Materials
Golden Paints Selection Chart
Finished Mural Protection
Environmental Factors
Definitions
Mural Procedures Quick Reference

Surface Considerations

Careful selection of surface for mural work should be done first. The ideal situation would allow the artist to work on a smooth surface.

If the mural is not an existing structure, but will be constructed specifically for your artwork, carefully consider the choice of building materials to be painted. For example, there are many plywood grades available. MDO plywood (Medium Density Overlay), often referred to as "sign painters board", is a high-quality plywood that is made to be used outdoors. It could also be attached to a wooden, metal or brick wall before painting a mural. This would offer the artist a surface that is clean, smooth and can be painted indoors in the studio, rather than on a scaffold at the mural site.
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Surface Preparation

Painting an Existing Surface:
If factors dictate painting directly onto a pre-existing surface, preparation is a major factor in the longevity of the mural.

Surface Cleaning Considerations:
The artist must be concerned with the nature of the surface to be painted. If the surface is already painted, then consider what kind of paint it is and its physical condition (the artist should realize that any surface that still has the previous coating on it will not be as permanent as one that has been completely stripped and freshly coated). If the paint is a water-based polymer, chances are good that the GOLDEN paints will adhere sufficiently. If it is a high gloss oil paint (or of unknown materials), then it must be abraded (or removed) for good adhesion. If the existing paint film is deteriorating, then it is best to have it removed (sandblasted, power-washed, scraped, etc.). It is critical to wash any painted surface, even a newly painted surface, with soap and water prior to application of acrylic products. A major coatings manufacturer states that as much as 80% of all coatings failure can be directly related to insufficient surface preparation.

Previously painted high gloss surfaces can be cleaned and dulled in one step by using a household abrasive cleaner. Wash off completely with clean water.

Mold and Mildew:
Mold and mildew must be removed by hand scrubbing with a mixture of 1 part household bleach to 3 parts water. CAUTION: Never add ammonia or ammonia-based cleaners to bleach! Wear goggles and protective equipment while cleaning. After scrubbing with a brush, allow the solution to sit on the surface for 10 minutes before thoroughly rinsing off with clean water.

Cracks and Grooves:
If there are cracks and grooves in the substrate, the method for filling and smoothing these gaps will depend on the nature of the substrate itself. Artists should consult an area architectural coatings store for recommendations on the best product available.

Priming the Surface:
Once the surface has been cleaned, a primer coat will give better adhesion for the paint. One key feature to look for in a primer is whether or not it can be painted over by latex (waterborne) paints. This should ensure that the primer will be a compatible surface for the adhesion of GOLDEN waterborne acrylics.

To determine the best primer1 for a specific surface, we suggest artists contact a local supplier of architectural coatings. Such companies have extensive experience with priming the broad spectrum of building supplies, and typically have specific primers for the surface the mural is to be painted on. Their recommendations will also take the environmental concerns of the area into account. We have found that architectural and maintenance paints are competitively priced, meaning that a product that costs more than a similar product will typically perform better as well.

When painting on brick, concrete, or other masonry surfaces, it is recommended to also use a Masonry Conditioner that can be purchased from a commercial coatings supplier.
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Painting Materials

Acrylics are some of the most durable and accessible paints for exterior application, used by many artists for painting murals due to their lightfastness and weather resistance. They also form an excellent bond to masonry or cementous surfaces. Oil paints are a poor choice for painting on these surfaces since the alkalinity of concrete can destroy alkyd or oil products. Ethyl-Silicate paints form an excellent and permanent bond with brick or concrete; however, this system can be time-consuming and costly, and good working knowledge of the system is required as well. Solvent-based enamels are a good choice for durability, but manufacturer's pigment considerations are usually not the same as with a high-quality artist's acrylic paint.

Golden Artist Colors produces several lines of paint that can be used for mural work.

Selecting which type of paint to use is dependent on each artist's style and the surface to be painted. GOLDEN Heavy Body, Matte, MSA, Fluid and Airbrush Colors can all be used for mural work. The artist must determine if the texture will influence the way he or she paints. For example, if painting on brick, it will be tough to get a smooth line brushed on with the Heavy Body Colors as is. They need to be thinned with GAC 200 (which also increases film hardness and potential durability) or the artist may consider switching to the Fluid Colors. The thinner consistency will allow the paint to flow into the crevices of the brick. (Refer to the chart below for selecting a suitable paint line).
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GOLDEN Paints Selection Chart

Use this Quick Reference Chart for choosing GOLDEN Acrylic Paints for Mural Use.

Note: Mixing is recommended with the GAC 200 to increase adhesion and durability, which will thin thicker viscosity products.

Choosing the Right Colors:
Color selection is especially important to minimize fading. The GOLDEN Pigment Identification Chart lists the relative lightfastness2 and permanency ratings of all our colors. For maximum longevity, we recommend using only colors with a lightfastness rating of I and a permanency rating of Excellent. Please note: Cadmium pigments should not be used outdoors, as premature fading will occur.

Increasing the Durability of the Paint System:
Although GOLDEN Acrylics are optimized for traditional easel painting, the acrylic resin is somewhat soft for mural work, and should be modified with a harder acrylic medium to maximize durability. Adding GAC 200 also reduces the pigment load of the paint mixture, making the paint more binder rich, which extends exterior lifetime. This is especially important if the artist chooses not to topcoat with a varnish. To improve the exterior durability of the paints, blend in 1 part GOLDEN GAC 200 for every 2 to 4 parts paint.

For exterior spray application, using GOLDEN Fluid Acrylics thinned with Airbrush Medium will be the most durable option. GOLDEN Airbrush Colors can also be used, but they should be blended with GOLDEN Airbrush Transparent Extender and given a protective finish to increase durability. In spray application, the GAC 200 is not practical to use as it will thicken the paint, and interfere with sprayability. Another approach would be to topcoat the Airbrush Colors with a sprayable isolation coat using a 2:1 mixture of GAC 500:GOLDEN Airbrush Transparent Extender.
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Finished Mural Protection

A final topcoat gives the mural more durability from the environmental factors.

The mural artist has a few choices on how to provide additional protection to the finished mural. One option is to apply an artist-quality varnish that is removable with various solvents, allowing for graffiti removal and general maintenance. GOLDEN MSA Varnish w/ UVLS is such a product, and more complete application information is provided below.

Another choice for protecting the mural is to use some of the various graffiti-resistant finishes that are commercially available. These range from protective wax coatings that are removed with hot water to the 2-component, solvent-based polyurethane coatings. They tend to have excellent chemical resistance, so that graffiti can be fairly easily stripped off without harming the coating. They also have excellent weatherability, and thus require less maintenance than some of the other choices. As we have not thoroughly evaluated these systems, we suggest you get all the information required from the manufacturers, and/or any mural groups that may share previous experiences, to determine the best choice for your specific application.

Listed below are application recommendations for using GOLDEN MSA Varnish w/ UVLS:

We recommend a 2-step system, the first being a permanent isolation coat, followed by GOLDEN MSA Varnish, a removable varnish. The function of the isolation coat is to physically isolate the paint from the removable varnish (which makes varnish removal much easier and safer to the paint surface). An isolation coat also develops a thicker layer of acrylic, which will give better protection and durability and serves to unify the acrylic paint layer.

Isolation Coat Application:
Choice of isolation coat should depend upon the artist's application method. Another consideration is the surface texture, as brush-applying the isolation coat and varnish over a highly textured surface can generate foam in the isolation coat. For spray application, we recommend a 2:1 mixture of GAC-500 to Transparent Airbrush Extender. For brush application, we recommend a 2:1 mixture of Soft Gel Gloss to water.

Varnish Application:
After the isolation coat is applied and has thoroughly dried (1-2 days, but no more than 1 week for maximum performance), GOLDEN Mineral Spirit Acrylic (MSA) Varnish can be applied. The MSA Varnish w/ UVLS is an excellent choice as a protective finish. This product provides a tough, durable surface that increases resistance to moisture and pollutants. The UV stabilizing system will serve to reduce the destructive potential of ultraviolet radiation from the sun, thus enhancing the life of the system. This varnish is removable in mineral spirits, turpentine and various aromatic solvents, which is a useful property for either graffiti removal or maintenance purposes. Spray application is recommended (1-3 coats).*

Note: Breathability is important to the successful adhesion of acrylic products. If a mural encompasses an entire wall made of a masonry product, it is advisable to apply thin coats of MSA Varnish. This will allow interior condensation and evaporating solvents and out-gassings to escape (some artists recommend leaving an uncoated breathing space near the edge of the mural as well). If this is not done, it may lead to premature adhesion failure between the coatings and the surface.

GOLDEN only recommends its MSA Varnishes for exterior work, NOT the Polymer Varnish. Polymer Varnish does not have the same exterior durability as the MSA.
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Environmental Factors

Temperature, precipitation, and humidity should be considered during application.

Temperature:
Ideally, the temperature during application should be between 65° and 75°F. Higher temperatures can affect application, especially the evenness of the paint films, as they will dry much more quickly. Colder temperatures can harm the film formation of practically all of the coatings, and the absolute minimum temperature is 50°F. Caution should be taken at temperatures approaching this threshold.

Precipitation, Humidity and Moisture:
Avoid painting in adverse weather conditions, such as rain and fog. This addition of moisture can effect the drying times and will re-introduce water back into recently dried paint films. It is especially important to allow proper drying/curing time between the isolation coat and the varnish layer. Trapped moisture can severely hinder adhesion, and can cause clouding of a clear layer. It should also be noted that sufficient drying time should be allowed between washing the surface and the initial primer or conditioner coats. Architectural coating manufacturers recommend waiting 30 days after washing concrete and cinder-block surfaces under normal drying conditions.

High humidity can also effect the mural’s drying times similar to the conditions cited above for precipitation. As the relative humidity climbs above 70%, more caution needs to be taken.

Definitions

1Primer - a coating that seals and prepares the surface to accept the subsequent paint layers.
2Lightfast - ability to withstand color change due to exposure to light.

Mural Procedure Quick Reference

Use this Mural Procedure Quick Reference to decide course of action.

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