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Aircraft painting - far more than outward appearances

Business aviation’s completion and refurbishment sector forms an umbrella for a great many individual divisions. A standard completions company normally has its hands in MRO as well, as a lot of the aspects of refurbishing involve maintenance-related work, such as avionics upgrades, fuselage and engine inspections, cabin improvements and general airframe maintenance. Consequently, this OTF report will focus on one aspect: aircraft painting. Although most often perceived as a means to make your aircraft look nice, for aircraft painting experts it is exactly the opposite. Jason Zappa Janse reports.

Considering that there may be many moments in the life of an aircraft when a paint job is required, this article will approach the concept of when an aircraft is in need of a repaint due to its years of activity. From this perspective, the following aspects can be considered as the basic steps of a standard paint procedure.

You name it, they should have it

Firstly, you need to select a company. It’s important to make sure that the company you choose is capable of matching your exact needs. For whatever reason you decide to bring in your aircraft, during that process there is a very real chance that you will encounter not only common corrosion effects, but potential signs of actual airframe deterioration. As indicated, refurbishing and MRO go hand in hand, so it is advisable to find a facility with a broad services portfolio, in case unexpected obstacles arise.

“Not every agency can do what the customer wants, so choosing the right facility is very important,” says Lufthansa Technik Director of Aircraft Painting Services Georg Fanta. He adds that a company should meet your required quality. Besides that, it’s crucial to keep the company’s slot availability and price in mind.

Attention to detail

When you have selected a company, you can start on the design. Fanta considers drafting a design as something that should be done with care. “Not everything that looks good on paper looks good on a real aircraft,” he explains.

When a design is finalized, a quote will be put together for the entire process. How do you compare price X from company A with price Y from company B? The answer is attention to detail. Quotes may not always say which aspects of the painting process are actually included and which not. Keep third-party designers, maintenance costs and other, perhaps less evident aspects, in mind. And take the time to evaluate your quote to know what’s precisely in there.

Acid, etch and Alodine

After this, the actual hands-on process commences, starting with a thorough inspection of the aircraft. “Your finish is only as good as your foundation,” comments Nicholas Lopez, paint department manager of Kirkland, WA-based Greenpoint Technologies. “The existing paint coatings must be chemically, if not mechanically, stripped from the aircraft in order to identify any potentially hidden damage under the surface.” Lopez considers chemical surface treatment of the aircraft as one of the most important aspects of aircraft painting. There are various chemical surface treatments, but a common one is the so-called “acid, etch and Alodine.” Among painting experts, Alodine is both a brand and a way of referring to chromate conversion coatings on an aluminum substrate. “Alodine provides both a passive surface layer that resists corrosion and a base for bonding the paint to the aluminum,” Lopez explains.

For Lopez, sanding is the next thing on the list. “Sanding the primer surface properly [is necessary] to reduce any skin imperfections [and ensure] maximum inter-coat adhesion,” he states, and he’s not the only one. “All composite surfaces need to be sanded,” acknowledges RUAG Aviation Director Interior & Exterior Services Carsten Matthiesen. “After that, all sealants need to be removed as well.”

Once the sanding has been completed, the aircraft is ready for the application of a primer. Among the various existing kinds of primers, it is possible to distinguish between two major systems: chrome and chrome-free.

Mean vs green

The use of the more traditional hexavalent chromium primer dates back to 1936. Since then, this kind of primer has served as the primary means of corrosion protection in the aviation industry. However, due to its environmentally unfriendly and carcinogenic nature, the US Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA) have both introduced regulations for tolerance levels. A 2006 OSHA mandate obligated a 52% reduction in permissible exposure limits to all hexavalent chromium. In other words, these so-called “green” primers, despite their name, are not completely chrome-free.

Consequently, paint experts sometimes still rely on hexavalent chromium primers. “These decisions are based on how these primers have performed over the years, compared to new products in the market,” explains Duncan Aviation Senior Completions and Modifications Sales Representative Nathan Klenke. “Hexavalent chromium primers have been used in the industry for years, but we still see corrosion happening,” he continues, “as opposed to chrome-free primers that have only recently been introduced, but don’t perform [as well] as the primers that do contain chromium.”

The main problem in the development of chrome-free primers is that, during the so-called “salt-spray” tests to verify the resistance of the primer against corrosion, chrome-free primers would only endure for an average of 2,000 flight hours, compared with 8,000 resisting hours of hexavalent chromium primers. “Of course, this wasn’t acceptable for the industry,” Klenke continues. “At present, there are chromium-free primers that resist up until 6,000 hours – still not as good as the chrome-containing primers, but we’re getting closer.”

The art of chemistry

Although the chrome-free primers’ performance might not be as high as that of the hexavalent chrome primers, the “green” process eliminates the normally required acid, etch and Alodine, replacing it with a pre-coat wash, before applying the chrome-free primer. “Basically, it’s giving the airframe a good, soapy scrub-down,” Klenke summarizes.

Still under development in the military aviation sector is a magnesium-rich primer, which shows promising performance results – higher than hexavalent chromium primers, going up to 12,000 hours of resistance during salt-spray tests. The mag-rich primer works in the same way as the chromium primer – if corrosion begins to take place between the skin of the airframe and the primer, the primer starts to sacrifice itself, to which the corrosion is being absorbed by the magnesium, rather than the corrosion affecting the aluminum.

Klenke goes on to say that it’s a process of constantly researching, testing and looking for the particular primer for that particular aircraft. “Importantly, 99% of all of this research is, or at least should be, geared toward the protection of the aircraft, while the perception is to make the aircraft look good,” he emphasizes.

Cascade effect

The introduction of EPA requirements was one of the many significant changes that affected the paint industry as a whole. It brought some heavy consequences. “Each time a system is being changed, the different combinations of primers and paints still need to chemically bond together to actually hold,” says Klenke. “The challenge is to find that exact combination. With a cascading effect, you have to ask yourself the following questions: With what has the aluminum of the aircraft been prepped, that allows the primer to stick to the aluminum? What kinds of chemicals does the primer contain that allows the top coat to stick to the primer? On top of all this, you need to put this on an object that expands and contracts through pressurization, flexes during landings and turbulence, goes from 100° on the ground to 20–30° below at high altitude,” he adds – an opinion shared by many.

“Aircraft painting is considered corrosion control,” Lopez further explains. “It’s your protection or insurance against the elements of the environment. It must hold up to the rigors of extreme temperature and high speeds. Certain aircraft have aspects [that are] directly affected by the weight of the paint, edges of the striping, or dark colors on heat sensitive and composite areas.”

Matthiesen adds to this: “The main variables during the application process are temperature, humidity, and air circulation. Furthermore, each paint batch might have a slightly different processability and appearance.”

Altogether, general opinion among experts considers the perfect result of a paint job as a balance between having it look exactly how the customer wants it, perform (protection-wise) to its optimum level, and remain easily maintainable after delivery. However, all agree that, regardless of its perception to solely please the eye, or merely serve as corrosion control, paint remains a kind of art.

Occasional touch-ups

When do you know your aircraft is due for a repaint? Well, it’s quite simple. When the effects of corrosion are actually visible, you’re too late.

“It’s not written anywhere,” says Klenke, “but what our experience tells us is that in five to seven years it’s advisable to consider a repaint, or at least a paint check. Of course, this greatly depends on the [number] of flight hours an aircraft logs during that timeframe."

That remains a variable, however, due to a trend that has arisen in recent years. Paint jobs are perceived increasingly as of less importance than, for instance, maintenance checks. Painting has become what Klenke calls a discretionary spending item, along with interior and avionics upgrades. The result is that paint checks are postponed to 10 to 12 years, corrosion effects are more extensive and may have spread out further than they should, making the costs of a now necessary paint job greater than they could have been.

If you really want to avoid high costs and long downtimes, occasional touch-ups are highly recommended, as opposed to a full check every 10 years. As an aircraft owner, paying attention to detail doesn’t only count for the paint shop – if you value your aircraft, it’s your job to keep an eye on any indications of age and wear.

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