mobile?Metal finishing Q&As since 1989
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Coating aluminum sculpture
Q. Hi, we commissioned an outdoor sculpture cast in aluminum A356.2. It absolutely can not be bronze because we are concerned that bronze would be stolen for copper. However we are also concerned about corrosion and durability of aluminum. The sculpture is very ornate, with many intricate details. I do not yet have a picture but I am including a picture of another similar art-deco panel with comparable level of detail. It will be placed outdoors in mild weather similar to Rome or Madrid -hot summers, mild winters, seasonal rain in Spring and fall. It will have almost no maintenance at all, and we would like for it to be in good condition for decades, hopefully up to 100 years.
We were considering anodizing (hard) but we were told it can not be done because of the level of details - supposedly that will cause uneven color. Is this true? An alternative of powder coating or sealing seams to be much less durable (estimated lifetime of under 10 years). If anodizing is not possible and not recommended, then is there any other solution! Thank you!
- New York NY
May 15, 2024
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A. Hi.
No finish will last 100 years but some should last, say, 30 years.
I don't think anodizing is ideal and the appearance would be mottled and gray even if everything worked out well. My preference would be:
Kynar water-base fluoropolymer coating if you can find a licensed applicator to do this relatively small project. It is available in aluminum color and other colors, and is a fairly simple 2-coat system. Start at kynaraquatec.arkema.com.
If you can't work that out, then I'd suggest chromate conversion coating the aluminum. Then priming it (electrocoat priming similar to what is done on auto bodies would be best if you can find a shop to do it). Then polyester powder coating.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
RFQ: Thank you very much! That is very helpful!
So if I understand correctly, the option A (Kynar fluoropolymer coating) would last around 30 years outdoors?
And option B electrocoat priming + powder coating: how long should that be expected to last outdoors ? Is that essentially what people mean by"powdercoating or is that a specific type?
And would both A and B involve applying a thin layer of paint (either powder or liquid) that will dull the surface details somewhat? Is there anything at all that will minimize how much details is covered up and also achieve a few decades of longevity ?
Also, does the specific aluminum allow matter?
- New York, NY
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A. Hi again.
Kynar is the most reliable painting system for long term anti-fade. It's usually guaranteed for 15 years, with an estimated life of 30 years. When applied on metal it will be two rather thin coats totaling about 0.004", so I don't think this will obscure any detail.
Powder coating per se is just a way of applying paint-like coatings by giving the item a static charge then spraying it with a fine powder which will adhere due to the item by static attraction; then it's put into an oven which melts the powder into a thin sheet. Polyester or polyester-epoxy resin systems are used for outdoor exposure. The electrocoat priming is not an inherent part of powder coating but it would increase its durability and life. Again, none of this stuff is really thick, the total system again would be about 0.003". Chromate conversion coating is a good corrosion-fighting prep for aluminum.
Powder coating is good but cannot be as fade-free as Kynar because fluoropolymer coatings involve chemical bonds which sunlight cannot affect, whereas sunlight can eventually cause other dye systems to fade.
The aluminum alloy matters greatly, but since the sculpture will be cast, you are limited to casting grades of aluminum, and they will not anodize attractively. If it were practical to make the artwork from billet aluminum or wrought aluminum it would be possible to anodize it to an attractive aluminum color finish. Cast aluminum will always be grayish and mottled. If you were to "hard anodize it", it would be a deep charcoal gray but probably inconsistent.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
A. Try NanoTech metal coating (USA product www.nanotechcoatings.com/products/metal-coating/) - little bit expensive but probably worth the investment. Hope it helps and good luck!
Goran Budija- Cerovski vrh Croatia
May 18, 2024
Q. Thank you very much! This info is very useful! Now that we know that anodizing is not recommended, we will try to inquire about Kynar fluoropolymer coating. We hope we can find somebody licensed around/outside New York City.
Another question: we were told that another possibility would be to darken/blacken the aluminum, and then seal it. Is there any type of sealing product that would be transparent (show the actual aluminum color) but last a few decades? So something like Kynar that you suggested but that has no color of its own, show the color of aluminum but lasts a long time?
And one more question: how long would the best possible powdercoat last?
These panels will be placed outdoors in another country where we do not live in, and you can assume that they will not get any regular upkeep or maintenance, and in any case we would like them to outlast our lifetimes. That is why we need the coating to last as long as technically possible to delay corrosion.
- New York NY
May 20, 2024
A. Hi again.
There are countless "clear coats". Goran mentioned one, our supporting advertiser everbrite.html offers others, automotive two-part clear coats are widely available, UV-curable coatings similar to fingernail lacquer are available -- but none of these will last decades let alone lifetimes.
I doubt that Kynar can be done in clear, although you should ask them.
But what probably could be done is clear chromate conversion coating, followed by a clear electrocoating or even a tinted translucent electrocoating (inquire of Clearclad Coatings, Harvey IL, or Molecular Technology. Wolverhampton England), followed by clear powder coating.
Anecdotally, I have an expanded metal patio set which was electrocoated and powder coated and it survived 25 years on a deck on a salt water lagoon, including several days submersion in the stormwater of Superstorm Sandy, before needing repainting. With luck, 30 years before re-coating is probably possible. With still more luck, even longer.
Logistically, however, lining up someone to do chromate conversion coating, then clear or tinted electrocoating, then powder coating may prove difficult. I think Kynar, in a nice shade that resembles aluminum, will prove easier to arrange.
Some years ago finishing.com arranged coating and corrosion testing of a good number of coatings for the International Zinc Association. We naturally tried to test all the "silver bullet", "magic" coatings we could find ... every one failed quickly. Some day some "magic" coating will prove me wrong and will reliably stop corrosion, but for now I'm not a believer. Automobile designers try their best to come up with clear coatings and decorative coatings for their cast aluminum wheels, but in the rough service that wheels face, I rarely see any that look really good after 5 years. But see if you can find a wheel coater and learn anything from them -- hopefully other readers will chime in.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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