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Maintenance

Cover up

There are different requirements for paint systems on the production line and in a maintenance facility. Ian Harbison investigates
 

Look at a new aircraft just off the production line and ready for delivery, and the decorative paint system will have been based on the manufacturer’s own specifications. These will call for paint shop temperature and humidity controls and an environmental protection system.
   
Fast forward a few years and the same aircraft is about to transition to a new owner. It is located at an MRO rather than a manufacturing facility, with a rather less sophisticated paint bay, and a different approach to refinishing is needed, including a range of materials  more flexible than OEM materials.
   
A new specification was required. AMS3095 was launched in 2001, and later revised to AMS3095A in 2004. This was developed for the industry by SAE, says Julie Voisin, Global Marketing Manager at Sherwin-Williams Aerospace Coatings, with a committee formed of engineers working in the aircraft and paint manufacturer industry. To obtain the AMS status involves extensive product testing by independent specialists.
   
Essentially, explains Richard Giles, Global Technical Service & Training Manager for Aerospace Coatings, the new specification took account of both Airbus and Boeing production requirements, as well as base chemistry characteristics such as corrosion prevention, resistance to ultra-violet radiation, flexibility to avoid cracking, gloss, longevity, colour fastness, repairability and ease of application.
   
This completely altered the way that all AMS qualified paint companies could assist aftermarket customers. Currently, there are over 50 different paint systems from the leading manufacturers (Sherwin-Williams has 10), incorporating primers, topcoats and base coat/clear coat solutions. Incidentally, that same independent testing process must still be followed for new AMS products today, and, says Voisin, it is very thorough – requests for changes are not unknown. „


Paint is generally delivered to a facility on a ‘just in time’ basis. The colour and volume will have been predetermined, but adaptions to the chemistry can be made to ensure good performance in different conditions. Most often, this is related to the drying time, accelerating it for a cold and damp US hangar or retarding it for a hot and humid hangar in Asia, and that is one of the biggest advantages of AMS3095A for the aftermarket, says Giles.
   
He adds that one area where is a divergence between the Americas and Europe is on the touchy subject of hexavalent chrome for decorative areas on aircraft. Identified as a health hazard, it is gradually being replaced by non-chrome systems. Europe is currently ahead in the process.
   
This is also a reflection of the differences between Airbus and Boeing.
   
Boeing has traditionally used alodine, a chromic acid conversion process that leaves a corrosion-resistant film on aluminium surfaces, followed by a chrome or non-chrome primer, then a topcoat.
   
Airbus, on the other hand, use wash primers, which provide a thin coating of phosphoric acid in a chromate solution to the provide corrosion resistance, and provide an adhesive base for the next coating, a urethane primer, which may or may not contain chrome.

 

Giles makes the point that, when an aircraft is stripped back to the bare metal for a heavy check, it becomes a blank canvas. This has a seen an increase in the use of non-chrome pretreatments and epoxy primers on Boeing aircraft.
   
From a user’s point of view, Randy Johnson, Director Corporate Aircraft Services at King Aerospace in Addison, TX, says that, in the past, the AMS3095 meant little to the corporate aviation refurbishment business.
   
However, since the company has increased its MRO capabilities and started doing maintenance, paint and modifications to Boeing and BBJ aircraft, it has found that the AMS3095 is a very useful system that is accepted by Boeing.
   

He adds that it is possible to achieve pretty much the same results with products that have achieved AMS3095 approval as most of the OEM standards, but there are some variations of quality in some of the different systems, and work has to be done to find the AMS3095 system approved that works for a aprticular paint facility.
   
As King Aerospace is quality-driven, key performance indicators for coating systems are corrosion protection, adhesion, durability, laydown ability (no orange peel) and gloss.


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