Selectively Strippable: Innovations in the Repainting Process

PPG Aerospace-PRC-DeSoto has introduced a selectively strippable paint system that allows you to remove the topcoat without removing the primer. The selectively strippable system is comprised of three layers: a chromate-free high-solids primer, a chromate-free intermediate coating, and a high-solids polyurethane topcoat.

The primer is formulated to provide corrosion resistance on metal structures. The intermediate coating acts as a barrier and allows the removal of the topcoat for repainting without affecting the primer. The topcoat features increased flexibility and gloss and color retention so the plane looks better longer, according to Alan Schoeder, global segment manager, commercial aerospace coatings, PRC-DeSoto. The topcoat allows airlines to extend the repainting cycle by up to two years compared with conventional topcoats.

“We’ve estimated you can save up to two-and-one-half days on a wide-body using this system just for downtime,” Schoeder says, “not counting chemicals and primers. Savings can range from $100,000 to $150,000 per day. It’s a range based on where it’s located. At the high end at $150,000 a day that’s $375,000. Now, $375,000 for an airline that’s bleeding cash is a big deal. An airline may paint 40 aircraft a year and if half are wide-body, 20 times $375,000 is big money.”

Based on information from customers using the selectively strippable system, there are also savings in materials. The system requires less stripper to take the paint off the aircraft, fewer chemicals for surface preparation, and there is no need to use cleaners, brighteners, sealants, or primers. This can amount to estimated savings of $5,000 to $20,000, according to Schoeder.

Schoeder relates the advantages to maintenance personnel: “the system reduces stripping, it eliminates chromate residue, and the primer and sealant don’t have to be reapplied,” along with the time savings mentioned earlier.

Environmental Concerns

“There’s a tremendous amount of pressure on the airlines and painters,” Schoeder says, “pressure from OSHA and the Environmental Protection Agency (EPA) here in the United States, to reduce chromate exposure and reduced chromate use in the application and removal of paint on an aircraft.”

The removal of chromates has been done in automotive and bridge painting applications which also have tough environments. “But they’ve had a lot of trouble removing the chromate that’s used for corrosion resistance,” Schoeder says. “There’s been some difficulty doing this in aerospace. The problem is that the chromates give some unique protection for the difficult corrosion tests that are run.”

“With an airline such as Singapore Airlines that’s getting the lion’s share of the early A380s, it won’t have to worry about the chromates in the stripper residue, and the benefits include saving primer, not having to worry about applying a chromated primer, and removing the toxic exposure for the painter and the people that are 100 feet away. It’s another big deal.”

When applying a chromated primer you have to be aware of how the area is going to be affected as it’s difficult to control. For example, in Singapore where the weather is hot, humid, and rainy, a hangar door to keep out the cold isn’t necessary. Spraying an aircraft creates a yellow cloud of chromated, pigmented primer overspray that will come rolling out of the hangar and can affect a large area. While painters are protected with personal protection equipment, maybe someone a couple miles away might not be. According to Schoeder, “Most organizations especially in Germany and Japan are saying they don’t want chromates at all. If you have chromates you have to have special ways of containing it to make sure it never gets outside a hangar.”

And then there are disposal issues. The chromate in traditional strippers is a problem for the sewage companies as they have certain kinds of bacteria that can clean up the sewage. And if you put the chromated material in the waste stream it’ll cause a problem. If you can eliminate the chromate it’s much easier to dispose of this residue, Schoeder says.

Testing Composites

As of right now the selectively strippable system hasn’t been approved for composites, according to Schoeder. “For the most part the strip and repaint process is being used on aluminum surfaces. The reason that Boeing and Airbus are so enthusiastic about this system while designing and testing their new aircraft is because of the larger amounts of composites. They want to be able to apply the stripper and have the intermediate coat on the whole aircraft including the composite parts.”

Right now airlines mask off areas that are composites that have the selectively strippable coatings on it. Composite areas like the vertical fin and body fairing don’t have to be stripped but they need to be sanded down. The airlines are waiting to get permission from the aircraft manufacturers to strip the whole aircraft including the composite areas. “The issue with composites is you’re not allowed to have strong chemicals like strippers come in contact with them as it can change the tensile strength, all the things that make it work structurally,” Schoeder says.

The selectively strippable system uses a weak stripper, one that has a weak activation agent. It could be acid, amine, or hydrogen peroxide. The normal levels shouldn’t come in contact with composites but if the level is reduced far enough it would allow the paint to be stripped off the composite surfaces.

“The theory is if you can take a container, say a quart can, that has the stripper in it and take some of the composite material and leave it in there for a certain amount of time, take it out, test it, and make sure it wasn’t affected by the stripper then you would know that even if there was a break in the paint film it wouldn’t ruin the composite if the stripper came in contact with it,” Schoeder says.

Manufacturers such as Boeing and Airbus want to go with a paint system that can be taken off with a mild chemical stripper on all parts of the aircraft including the composite parts, Schoeder says. And the airlines want a chromate primer so manufacturers can apply it in a very controlled environment and the airline would never have to respray the chromate primer. All they have to do is reapply the intermediate coat and topcoat.

“Airbus is committed to making information available in early 2006 for airlines to safely selectively strip composite parts,” Schoeder says. “The idea is you can strip the paint off composite parts if it has an intermediate coating; however, you have to use these particular strippers from the major stripping companies.” The aircraft manufacturers are working closely with coatings suppliers to find the best solution.

As the amount of composites used on aircraft increase with the emphasis on improved performance and fuel efficiency, the market is ready for a product that will fit the new structural requirements. The time and money saved by not having to sand and being able to use a weaker stripper in the process offer advantages to manufacturers and maintenance departments. The number of aircraft per manufacturer using the system, according to Schoeder, includes 200 for British Airways, more than 50 for Boeing, and more than 100 for Airbus. The topcoat alone has been applied to more than 10,000 aircraft worldwide.