An exterior mural is likely the most extreme test for lightfastness and weatherability of a paint system. One of our most popular Application Information Sheets, “Painting Exterior Murals” is the culmination of decades of experience and research. If you have never read it, please do. You will soon realize a mural is only as durable as its components and preparation is vital for long term success. Naturally, a key to this success comes from selecting paints best able to withstand the worst environmental conditions.
GOLDEN’s acrylic paint lines are well suited for use as an exterior mural paint. They have an exceptional exterior binder coupled with pigments chosen for their durability. However, just being artist grade paint isn’t enough when it comes to an exterior mural.
“Fine”, you say, “I’ll stick to just using ASTM Rated Lightfastness I pigments.” That’s a great start, but the ASTM LF-I pigment rating system is intended for interior use applications without any direct sunlight. Outdoor murals have to be durable against UV, heat, moisture, alkaline surfaces and acid rain. Some pigments only reveal their true durability under a combination of factors; certain paint systems such as oils don’t perform well on many of the brick and block alkaline surfaces typical of murals. This is why we do not solely rely upon ASTM LF testing as the only indicator of success.
Artist paints also differ from house paints and other commercial coatings. In artist paints there tends to be a much wider range of pigments used and typically, they have higher pigment loads as well as some larger pigment particles, making them more likely to be exposed to the elements. We regularly suggest adding additional acrylic binder, such as GAC 200, into paints. This adds a harder binder to aid in physical toughness and promotes better adhesion as well. It also yields more paint to use on your mural. Overall though, once the initial concerns are addressed, we can paint using the same paints for the mural as we use for easel painting.
Don’t Go Changing
Color fading comes to mind first when we think about sunlight exposure. Fugitive colors literally can disappear from view, which has been an issue for centuries. But along with vanishing, there might also be color change. We measure this change with an instrument called a spectrophotometer, which specifically gives us readings of color change in ‘delta units’. The amount of color drift from the initial starting point is how we determine if a color is “lightfast” or not. A color may fade, darken or otherwise change its hue. Fluorescent pigments (actually a dye encapsulated in acrylic binder) are notorious for their poor lightfastness. Not only will they eventually fade severely, but they go through a very drastic change along the way. The once vibrant optically bright color can quickly become muddy brown. Fluorescents therefore are an obvious exclusion for the exterior muralist palette, but there are others that may come as a surprise.
Pigments rated ASTM Lightfastness II are completely suitable as artist grade paints, but should be avoided for any exterior use. They include Dioxazine Purple and Nickel Azo Yellow (a key component of our Quinacridone/Nickel Azo Gold). Hansa Yellow Light is also a comparatively less lightfast pigment amongst the other yellow arylide pigments.
Cadmiums are pigments associated with many strong opinions. Yes, they are heavy metal pigments and should be handled with care. Yes, they are an artist staple with rock solid stability for interior use – but therein lies the rub. When Cadmium Yellows come into contact with UV, heat and moisture at the same time, they can change from brilliant yellows to pastels. Cadmium Reds, while a good deal more resilient, are vulnerable to color shift under the same conditions and should therefore also be avoided for outdoor use.
Ultramarine. You didn’t want to see Ultramarine Blue on this list. We get it. We love it too. However, as Ultramarine pigments are exposed to mild acids, a chemical reaction causes the bright blue pigment to “chalk” and turn white. We saturated a test card with a vinegar based household cleaning solution to simulate acid rain and upon drying, the surface pigment changed to white.
The Crossroads Blues
Diarylide Yellow. 10 mil. Masstone and 10:1 tint. Unvarnished.
Left is unexposed, right after 3 year exposure.
Diarylide Yellow. 10 mil. Masstone and 10:1 tint. Unvarnished. Left is unexposed, right after 3 year exposure.
The Painting Exterior Murals Application Information on our website provides a list of “good” colors. It is best to think of these paints as “on the fence” for mural use. The colors between “Best Pigments” and the “do not use list,” typically includes colors that can be used for murals but only opaquely because pigments on the surface may change, but as long as there is more of the same color underneath, the change goes relatively unnoticed. For best results we always suggest creating your mural with dense, opaque paint layers regardless of color.
Use these colors with a heavy hand and add Titanium White to lighten the color instead of as a glaze or wash:
- Diarylide Yellow
- Hansa Yellow Opaque
- Naples Yellow Hue
- Turquois (Phthalo)
- Cerulean Blue
- Cerulean Blue Deep
- Quinacridone Red
Best Recommendations for Mural Colors
When it comes to the most stable pigments available, iron oxides reign supreme. This is why it makes sense to incorporate them into a mural. We may not realize how widespread these pigments are used by artists, as their common names may or may not reflect their actual base ingredients. When developing the mural concept – with the aid of prototypical maquette (perhaps the best planning tool a muralist can utilize) – try to see if you can work within a group of paints made from mostly Oxides.
Earth Colors
Earthy Inorganics often have a romantic name, evoking images of rolling Tuscan hillsides with mandolins playing in the background as Renaissance-era workers carefully fill a sack of the finest ochre destined for Vasari’s workshop. But wherever a vein of Limonite is found on the planet, it can be named PY-43, Yellow Ochre or Raw Sienna as we know it today. Take that iron oxide clay and heat it and you have Hematite, or what we know it as, Burnt Sienna. Regardless of their name, these are extremely stable and reliable pigments for mural use.
- Yellow Ochre
- Raw Sienna
- Burnt Sienna
- Raw Umber
- Burnt Umber
- Burnt Umber Light
Man-Made Oxides
Rather than rely upon veins of naturally occurring sources for specific earth colors, pigment manufacturers combine the base ingredients at certain percentages to create synthetic versions. We know them as “Oxides” or “Mars” pigments. A wonderful range of pigment choices have come from this process and they are as reliable as their natural inspiration. More recently, a new variation of these pigments has been given to the artist, where it’s been transformed from an opaque pigment into a transparent version. These transparent pigments are every bit as stable, and produce very clean and vibrant colors.
- Yellow Oxide
- Mars Yellow
- Red Oxide
- Violet Oxide
- Transparent Yellow Iron Oxide
- Transparent Brown Iron Oxide
- Transparent Red Iron Oxide
The Bright Side Of Oxides
We tend to think of oxides as being just a range of browns, but there are actually many bright, vibrant colors ranging from yellow, to green and into blues. When iron is swapped out with chromium and/or cobalt, the results are stable and stunning.
- Chromium Oxide Green
- Chromium Oxide Green Dark
- Titanate Yellow
- Cerulean Blue, Chromium
- Cerulean Blue Deep
- Cobalt Green
- Cobalt Turquois
Ain’t Nobody’s Bismuth
Bismuth Vanadate Yellow, an opaque greenish-yellow, is one of the more recent color entries into the GOLDEN lineup. As was previously mentioned, Cadmium Yellows are not recommended for use on an exterior mural. Hansa Yellow Opaque can still be too transparent for some artists, and Titanate Yellow is quite pale, eliminating it as a viable option for many color mixtures. Enter Bismuth Vanadate Yellow, which fits nicely between Cadmium Yellow Light and Cadmium Yellow Primrose, yet has the permanence of Yellow Oxide.
- Bismuth Vanadate Yellow
The Royal Family
Pyrrole pigments have been on the art market since the early 90’s, as an excellent alternative to the heavy metal Cadmiums. Not only are they similar in opacity but they are vibrant and clean mixers with a history of being solid when used outdoors. No other family of bright oranges and reds can even compare.
- Transparent Pyrrole Orange
- Pyrrole Orange
- Pyrrole Red Light
- Pyrrole Red
- Pyrrole Red Dark
Paint It Black?
While all of the black pigments used by GOLDEN are perfectly suitable for exterior use, their dark value can result in something that catches new exterior muralists off guard when exposed to large amounts of south facing light. This leads neatly into a discussion about color choice for murals. Studies conducted by the Lawrence Berkeley National Laboratory show that, on a 55° day in Los Angeles, black acrylic paint got up to 142° F versus white paint, which peaked at 74° F1. Granted, these were on roofs exposed to the overhead sun, but it still gives a dramatic example of just how much the absorption and conversion of solar energy into heat increases the surface temperature.
What does that mean? If you use large amounts of black in a mural, you can create hot spots on the wall. The impact to longevity of the mural is unclear but any build-up of heat adds additional stress to paint.
White Light / White Heat
In the case of an exterior mural, many times it begins with a white basecoat (White Gesso with additional acrylic medium or an all acrylic exterior house paint). Titanium Dioxide has been the darling of the house paint world for many decades and for good reason.
While there are actually many kinds of TiO2 pigments available, the ones used by GOLDEN have a proven record for exterior applications. Many commercial grades of Titanium Dioxide pigments are prone to a reaction with UV light, which can create free radical formation and lead to chalking of the paint film. Our pigment was specifically chosen for its protective coating which reduces, but not eliminate, the UV degradation. It’s an extremely durable pigment so use it and its warmer cousin Titan Buff (unbleached Titanium White).
Zinc White can be used in a mural but its highly transparent nature can be an issue when coupled with Phthalo and Quinacridone, so use with caution.
- Titanium White
- Titan Buff
- Zinc White
Shine On You, Crazy Diamond
The longevity of Iridescent and Interference pigments can be confusing. Although they are listed as “permanent” they are not able to be measured for ASTM Lightfastness rankings. Why? Because the color spectrophotometer can’t read a reflective paint the way it does with regular pigments. Fortunately, we have some equipment that can accurately scan for color change, and it’s called the human eye. It’s the best device for this task, and we always have more than one pair of eyes take a look at the exposed test cards to see how they fare. After assessing accelerated weather testing – both UVA and Xenon Arc methods – and multiple long term South Florida real time exposures, we feel confident in their ability to hold up when used in an exterior mural system.
The only Iridescent color to avoid is Iridescent Bright Gold because it is a blend of mica and LF II Quinacridone / Nickel Azo Gold, not a pure mica pigment. Colors to use:
Interference Colors:
- Blue (Fine)
- Gold (Fine)
- Green (Fine)
- Orange (Fine)
- Red (Fine)
- Violet (Fine)
Iridescent Colors:
- Bronze (Fine)
- Copper (Fine) & (Coarse)
- Copper Lt (Fine) & (Coarse)
- Gold (Fine) & (Coarse)
- Gold Deep (Fine)
- Pearl (Fine) & (Coarse)
- Silver (Fine)
Actual Metals:
- Micaceous Iron Oxide
- Stainless Steel (Fine) & (Coarse)
Natural Micas:
- Black Mica Flake (Small)
- Gold Mica Flake (Small) & (Large)
- Pearl Mica Flake (Small)
If we pair the colors detailed in this article with appropriate priming and subsequent sealing with a protective varnish, your murals should remain looking great for a long time to come.
Resources/Further Reading
http://spinoff.nasa.gov/spinoff2003/er_4.html
http://pcimag.com/articles/a-comprehensive-understanding-of-tio2-pigment-durability
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