Skylight systems are typically constructed with aluminum framing, as it is strong, lightweight, and easy to work with.
Aluminum also has the benefit of being less susceptible to weathering/corrosion than many metals, as it naturally forms an invisible, hard oxide “skin”, which reforms if scratched or worn away. When aluminum does corrode, it develops localized pits; aluminum does not discolor and stain when it corrodes (like rust on steel), but a natural (mill) finish will darken slightly over time. This mill finish offers decent durability if not exposed to harsh (highly alkaline or acidic) environments.
In practice, many environmental pollutants can be corrosive to aluminum: salt in coastal/marine locations or from de-icing roads/sidewalks, excrement from birds and other animals, acid rain, industrial pollution, automobile emissions, fertilizers, etc.
Aluminum is often given an anodized or painted finish, for additional protection and longevity, as well as for aesthetics.
Anodizing is a controlled electrochemical process that creates a thicker version of the natural aluminum oxide skin. While a mill finish will naturally form a skin that’s .004 microns thick, a Class II anodized finish is at least 10 microns (0.4 mil) thick and a Class I anodized finish is at least 18 microns (0.7 mil) thick. This extremely hard and durable aluminum oxide layer is integral to the aluminum below it (as opposed to an applied finish), so it will not chip or peal.
Standard anodized colors range from ‘clear’ (natural color) through a range of bronze tones, from very light ‘champagne’ to ‘black’. These integral colors are inorganic and will not fade or chalk. Premium palette options are also available, from the appearance of other metals (copper, brass, etc.) to bright, bold colors, via additives and dyes during the anodizing process. (It’s worth noting that white anodized is not available).
There are too many options for painted aluminum finishes to cover here. Broadly, we recommend powder coating over liquid painting for higher performance and lack of volatile organic compounds (VOCs). Specifically, we recommend thermoset (vs. thermoplastic) powder coatings using fluoropolymer resins (PVDF, vs. epoxy, acrylic, or polyester) as these finishes best suited for exterior architectural applications: they yield the best weatherability and UV-stability available. (Brand names include Kynar, Hylar, Fluropon, Duranar, etc.). We recommend painted finishes with at least 70% PVDF by weight; 50% is cheaper, but comes with a big performance drop. A 2-coat application (primer + color coat) is typical, but 3-coat (primer + color coat + clear coat / effect coat) is available at additional cost. You can specify either AAMA 2604 or 2605 as a standard for exterior painted finishes. 2604 is a typical, intermediate standard and will ensure good wear-resistance plus color and gloss retention for at least 5 years of exposure. 2605 is a more expensive, high performance standard for longer term protective and cosmetic performance.
Painted aluminum is available in a rainbow of standard colors (including various shades of white). You can also get custom/matched colors, extra bright colors, pearlescent or metallic effects, etc. or for a moderate upcharge.
In terms of cost, standard anodized and painted aluminum finish costs are usually fairly comparable. Anodized finishes are more durable and longer lasting, but have a more limited selection of colors/effects. Anodized finishes allow the aluminum’s natural appearance to show through – more for lighter colors, less for darker; the effect is beautiful, but requires care by the manufacturer and installer as damage, die lines, welds, etc. will show through. On projects that require bent/stretch formed components, component dimensions may preclude an anodized finish. Components cannot be formed post-anodizing as the hard aluminum oxide skin will crack and craze; components can be anodized after forming, as long as they fit in the anodizing tank!
WSD can provide separate interior and exterior finishes. WSD has even provided a different material, like copper, at the exterior in lieu of aluminum. (Care has to be taken to prevent galvanic corrosion from dissimilar metals).